pages old but i thought i should chime in:
if you're 99% sure it's being worn by a person who will have a good response, or you know what it is already, ask cuz it's fun. I ran into a guy at a wedding a week ago who was wearing a margiela memory blazer (or whatever it's called, where the buttons and pockets and shit are all not really there but are just bumps or shapes) and some crazy color-blob shirt so i asked him, knowing he was on some runway shit. it was a paul smith shirt from maybe a year ago, and yes, margiela. he appreciated it.
people ask me about mostly my sunglasses, and the answer, "japan," always ends the conversation real fast, but nobody walks away disappointed.