i've probably bought stolen stuff before. there's little way to tell. it's not like the guys selling stuff look and act like peter lorre in 'm.' they don't usually have a chalked 't' on their shoulder designating them a thief.
with that said, we're all thieves in one way or another. how many threads you think there are here bragging about the bike just purchased for pennies on the dollar from the guy that didn't know what he was selling?