Caneaddict....this is a great question as I've mentioned. I remember a few years ago, drawing up a mock draft at the start of what was probably the 1989 season and putting the Rice TB Trevor Cobb in high. My reasoning was that he had a bunch of yards and I hadn't seen him play. So when I read some literature about the games best draft prospect backs and Cobb was no-where to be seen, I thought it odd. But then when I saw him play and saw that the reason he had all the stats was because Rice just ran him and ran him and ran him, behind a big old OL, against some questionable competition at times and that he was undersized, that he didn't have great speed or hands and nor was he a great blocker, it became clear. Yet the guy won the Doak Walker. And this is where people get confused. But you are looking for translational abilities.
With that in mind, essentially it comes down to instinct when scouting. Repetition helps - watching a tonne of tapes, a lot of players, looking at their bodies - will they add weight, how do they look when they move, does the game seem easy for them, have they seperated themselves from the rest and if so, why? Can they stack up to NFL pounding week in and week out. Remember, college ball has 116 odd 1-A teams - that's a LOT of players. There is a lot less in the pros.
Take a running back - how quick are they, vision, can they pick up the blitz, can they SEE the blitz, what are their hands like, can they get down the field, can they see a hole, how quickly do they accelerate into that hole, take on a linebacker, what's the blocking like, are they inteference blockers, or can they actually stand someone up, whats their blocking technique, can they pick the right hole, how do they hold the ball, can they bounce back after a fumble, are they high cut, what are their legs like - thighs - how big, lower legs, how much power, do they change the ball to protect it when running, what are they like close to the goaline, what are the hips like, can they read blocks, are they patient, do they run north/south etc?, can they get to the edge, can they turn their shoulders upfield, how big are the hands, etc etc etc.
Same with the QB - look at Steve Taylor. He was a running QB who didn't have the arm or the delivery for the pros at that time, in an era where the drop back pocket passer was de rigeur. What are the mechanics like, where does the ball come from, how quick does it come out, whats the motion like, follow through, footwork? Do they step into the throws, are they flat footed, are they mobile, can they throw on the run, what's the motion like on the run, how do they deal with the blitz, can they see it, can they check out to the right play, how do they command the huddle, can they lead, how strong is the arm, can they sense pressure, can they remain calm in the pocket, can they step up, can they make all the throws, how do they work through their progressions, can they make yards with their feet, what is the body like, what are the tapes habits like, are they intelligent, can they stand up to a pass rush, does he take his eyes off the receiver and looks at the DE, can they take the sack without making the pick, etc etc etc.
When you can answer enough of those questions in the positive, you have something you can work with.
So why do some players develop in college and then cease do be effective in the pros or vice versa? A lot is systems, which I will touch on in a minute, but to take one of your guys, Troy Edwards - he was the ultimate system WR. Matched up a lot in the slot, La Tech moved him around a hell of a lot, created gimmick plays that were succesful for him and remember, he was an undersized guy playing essentially against inferior talent. He was simply not big or physical enough to get away with that in the NFL. He was playing corners that either didn't get their hands on him at the line or who weren't quick enough. That didn't happen in the NFL. Suddenly he's going against 6-1, 200lb physical press corners.
A lot too much emphaisis is placed on the Combine - far too much. People suddenly forget the body of work of a player through 4 years and go on one flashy 40 time and some catches in shorts and a t-shirt. When a player has a surprisingly impressive workout, the first reaction should not be to move him up three rounds on the draft board. It should be a re-evaluation. You mention Rice and Thomas - well Rice famously ran a slow time at the Combine, but he was drafted into a system that suited him perfectly - his ability after the catch, his speed over the first 5 steps, his route running ability and his hands, honed from years of catching bricks as a child. Look at the flip side - USC WR R. Jay Soward ran a 4.35 40-yard dash at the combine because nobody in the Pac-10 could catch him. People forgot that he was a flameout who ran questionable routes and had inconsistent hands. His drafting was the beginning of the end of Tom Coughlin in Jacksonville.
As for Thomas, size was less of an issue for him than it was for Edwards because there are ways to deal with Zach's size, which JJ did perfectly - he knew he could succeed behind behemoth DT's. As a WR you can't hide your deficiencies as easily.
Staying with the Combine, the type of drills a player excels in also can be telling. What do I mean? Well....a short shuttle is more telling about a DLmans ability than a 40-yard dash. People grade players all too often on the way he excels in tests that don't measure his ability to play his position. When I see the DL test results at Indy, unless the guy has run a 5.8, I could care less about the 40 time. What 'm looking at is the 5 and 10 yard splits. How quick he is in and out of the shuttle drill, which will measure how explosive he is over a short distance and how quickly he changes direction.
Take Chris Hovan as an example, a player we may go after in FA. Many teams had questions about his athleticism when he came out. When he performed the specific DT drills, those questions disappeared. All of the tests, the runs and jumps can be practiced and rehearsed. You can get better with reps. Athleticism is much harder to rehearse.
Again, you need to evaluate workouts differently. A great workout by an offensive lineman should be regarded more lightly than a great workout by a wide receiver. Why? Because athleticism matters much less to the former than it does the latter. Workouts also can be most revealing for tight ends. You could be seeing them do things that they have never done before because a lot come from run-oriented offenses. That's why a lot of TE's slip through the net. And then a lot of WR's end up doing the TE drills and all of a sudden you see how well they can block and they have the size to become a good TE/H-back. Take Denver's Jeb Putzier - he's the perfect example of this. At Boise State he ran patterns as a WR, but didn't block much. At workouts, he proved that he could get his hands on people, take good angles, use leverage. The fact that he was too slow to be a top end WR didn't matter. All of a sudden, Mike Shanahan has one of the best TE's in the game on his hand.
But it doesn't always work so easily as that Cane. There are reasons players don't always show their athleticism on the field that you have to take into consideration.
Sometimes the player is poorly coached in college. This is particularly possible with small-school players. Not denegrating the work of small school coaches, but that's sometimes just the way it is. Look at how many small schoolers there are in the pros and how they were drafted. That's not a coincidence. But you have to weigh up the competition and that's hard to do when each week the guy you like is playing Kutztown State and Grand Valley State etc. Take our own Lamar Gordon - I have a tape upstairs of his from college. Sent to me by the SID of the college, it has all his highlights and then a game tape of a particular game. You can see he dominates at that level and you can see why he was rated so highly. But you look at the guys he's playing against. Can he be that good in the pros? I don't think we still have that answer. Another good example was Ladanian Tomlinson. To my mind, he's the best back in the NFL, but there was plenty of fuss about him when he came out of TCU because of the level of competition that he played against.
Sometimes Cane it's the scheme used at the player's college that doesn't bring out the best in him. Take Philly's Jevon Kearse. He's a much better NFL player than he was at Florida. Titans coaches studied every snap he was in on for the Gators and worked out what he did best, in which situation he succeeded most, etc etc etc. They then eliminated all the plays he struggled on and moulded a defense around his particular athletic abilities and he has subsequently flourished. He was a great player at UF, but he got by on athleticism more than honed DE skill. Look at backs that come from pure run or pure pass systems. Often they struggle at the next level because they only know 1 discipline - either pass or run. So the runners don't know how to catch proficiently, they don't understand picking up the blitz, etc. And they have played behind big old roadgrading run blockers. This is typical in places like Nebraska - till last year - Wisconsin, Kansas, etc. When they get to the pros they have to learn the game almost from scratch. And it's tough because you are now playing with the elite. There are only 32 teams not 116. Instead of being the big fish in the small pond, you are now the small fish in the big pond. Now trying being the small fish and having to learn how to swim. Almost impossible.
Sometimes the player is not playing his best pro position in college. A good example of this is the Bears Jerry Azumah. A productive small-school back at New Hampshire, he was too small to be an every down pro back, so Dick Jauron turned him into a CB - 5'10, 195. It turns out to have been a good switch. Brian Urlacher - an active, big hitting safety in college, but the Bears thought he could handle the switch and it proved thus.
Sometimes it's the players on the team that the player is playing for in college that don't allow him to flourish. There is a well known story of when Bill Walsh, then working for the Bengals, went to see the workout of a quarterback for a Division III school. Walsh says that the team were struggling, people weren't making plays, but he saw something in this kid. So he took him to the gym, just him and this player and he got him throwing and he saw the arm and the quick delivery and the mechanics. He got him moving around and saw the atheticism and the footwork. So he pestered and pestered and pestered and finally he convinced the Bengals to invest a 3rd rounder on the kid. It was Ken Anderson who turned into a hell of a player.
So the bottom line is that experience comes with time. It's easy to see the big time big schoolers and whether they will be a pro player. But can you put on a Northern Colorado tape tonight and tell me why Vincent Jackson might be the next Terrell Owens? Can you put on a Louisiana Monroe tape and tell me which of their 11 starters is highly rated and why*? When you can do that - and it only comes with time and some of the factors above, THATS when you know what you are looking for!!
*It's safety Chris Harris BTW.
Hope that helps.
Boom.