A hipster is someone who is smart enough to talk about philosophy, music, politics, art, etc. with you all day long, but not smart enough to see how big of a tool s/he is.
The only sure fire way to tell if someone you're talking to is, in fact, a hipster is to ask them "are you a hipster?". If they respond no, and turn their casette player back on, you can be sure you're dealing with a hipster.
Person 1: I met this really smart girl with a goofy haircut, but I think she has asperger's.
Person 2: She's probably just a hipster.