We all read body language from time to time. We see someone sometimes, and we trust them immediately. We don't even know why. But then of course, we learn a little while later that they just turned out to be the ax murderer from the massacre down at the shopping mall.
Basically, whether we're successful when we try to read body language or otherwise, we're always trying to look for a few cues that give someone away. A smile that seems slightly out of place, a choice of words that seems odd for the person, a circumstance that seems to not entirely belong there – we all know so well how normal behavior goes that with a bit of practice, we're always able to catch out anything that's not entirely right.