Note the position of his body. If you like the person, he is often turned to you face, not the back. Wanting to understand all of what you say, this person will slightly lean towards you, hunched. In a sitting position, interested you people the whole torso (and hips and back) is deployed in your direction. Arms and legs not crossed. Making conclusions, try to be as objective as possible. Your interest in person may prevent you to correctly assess the situation.
Look at the picture on the wall in the time of the conversation with the man. And then look him in the eye. If you like the person, he will follow your eyes – just like you, take a look at the picture. (If no pictures, the paper clip would also work). Watching the man, do it quietly. Otherwise, his behavior is just a response to your undivided attention, and for the accuracy of the results of the experiment need immediate reaction.
Adjust the face, play around with facial expressions: slight frown, something to smile at some point freeze, as if in thought. See "mirrors" is he your facial expressions. Tap fingers on the table, note whether his fingers still or stirred, repeating your movement. It is human nature to repeat the gestures and facial expressions behind those who he likes.
Listen to what he says, what most readily respond, on what topics he starts a conversation. If the conversation on his initiative most often comes about that interests you both, it means that people want to like you, convinced that you're "soul mates".
You touch his hand, telling a simple personal story or anecdote, zoom in very closely. If you like the person, he will not clean his hand and moved away, otherwise, to pull away, trying to keep a comfortable distance.
Do not hesitate to help friends/girlfriends. Although from the outside become visible some aspects, they will not catch the various nuances which are noticeable only to you.