You can tell it's Manning just by way he moves and throws. No one moves and throws quite like he does. If it's an impostor, it's a 6'5 impostor who has studied Manning's mechanics in detail.
Seems like guys like Bob Kravtiz are seconding my speculation here about the release of this "amateur" video being a carefully orchestrated Manning plan, either to let Colts fans know he's progressing and thus pressure Indy to keep him, and/or to let teams know that he can throw the ball, if perhaps not 100% -- and he doesn't look 100% to me -- then better than it's been speculated he can throw.