There is no way to tell the difference in a subway station, even for an instrument as loud as a violin. Moreover, no one goes to the subway to listen to music.
If you're into this kind of music, you could tell, sure. He begin playing one of the hardest to master solo violin pieces of the repertoire. If I saw someone playing Bach's Chaconne on the street, I'd stop right away. There's no way street musicians would be able to play it like he did. Well, about a thousand people passed by, and seven stopped to listen. 0.7% of the sample isn't that bad. Plus, he made 50 bucks in less than an hour.
Yeah that's the qualm I've always had with that 'experiment' in a subway you're on your way to do something else, usually on a time crunch. Add on the noise and the fact that most people couldn't tell an amazing concert level performer from a simply good performer and the result is foregone.
I'm not saying I would have stopped, because I don't often, but I could tell. I have neighbors who are very into this sort of thing and from the few hours of exposure I've gotten through them, I would have known immediately that this was no average performance.
Also a violin as expensive as the one he was playing sounds pretty great in any environment.
