I used to do this thing at bars when I was an undergrad
Do you mean that you tried to guess whether a stranger was an introvert or an extrovert?
Yes.  All I had to do was watch him from behind
It seems pretty interesting making up stories about people by observation. However, you shouldn't assume that you know someone just by watching them. People are unique and complicated individuals, and it takes time to really understand them.
So?
Well if you make up something bad by the way someone looks you might really miss out on getting to know a nice person. Have you ever found out that you were wrong about someone after you approached them?