Whenever us humans are out in a social situation, we put out a whole range of verbal and physical indications of what we're feeling. Like a poker player, you can read a lot by the way a person is standing, the tone of their voice and selection of words, where their eyes are looking, and what they're doing with their hands.
Reading body language is harder than it seems - otherwise we'd all be picking up every night. But with a bit of practice, you can tell what a person is thinking without even having to ask them. You can tell what your girl is thinking, too, without confronting them. Are they giving you their undivided attention? Are they still totally into you? Or are they opening themselves up for a one night stand with someone else?