QUOTE(Robodoc @ Mar 23 2008, 12:16 AM)

Maybe I could see the reflection of the floor tiles (he was in the room with the chessboard pattern floor that you can see him in on youtube playing Chopin studies). Maybe he's just got a distinctive way of positioning and moving his hands. I don't know. I was sure, though.
Probably a mixture of those things. Also, could you hear him or was the sound turned off?
I sang in a service last year with 2 conductors and 3 organists (it was something with three choirs all singing together, so the guys from all three shared the jobs out between themselves). Because I was on the same side as the organ loft, I couldn't see which organists played which parts of the service, but I was able to tell, with absolute certainty, which one it was for some things because I recognised his playing style, just from the sound.
So, unless the sound was turned off, your ears were probably telling you as much as your eyes - maybe more.
I've got a friend who went to school at King's (he wasn't a chorister himself and isn't into that type of music at all). If I play him CDs of choral music, he can pick out which ones are from King's College, because of the distinctive accoustics of the building and the characteristic sound of the choir. (It's the building not the choir he's homing in on most because I played him one of King's choir recorded elsewhere and he said that if it was King's they weren't in the chapel. And he explained to me that he can hear the different echoes resulting from the narrow width and the long length of the building. To him, it's an aural 'fingerprint', which he just recognises without having to even think about it.)
He's not a musician. You are, so your ears are probably far more attuned to such details.
T.