He shrugged a bit, searching for a good answer, "Guess because you can learn a lot just by watching how a person presents themselves." There was a lot more behind that answer, but he figured it was good enough.
That was really something he learned working with Bruce. Besides occasionally getting vocally angry, Bruce was pretty good at harboring emotions. But everyone has their tics and little things that give them away. Even Batman. The skill was also useful with criminals and informants. Granted, it was hard to rely completely on just watching them and studying body language alone, but it sure as hell didn't hurt.