Step1
Know a little about the man whose body language you are interested in reading. Learn about the man's individual personality and his culture. In some cultures, there are different meanings attached to nonverbal cues. A Native American man may not look you in the eye, for instance, but this doesn't mean he isn't interested.
Step2
Realize that the place where you have the encounter with the man may influence the body language. If you are at a formal gathering and the man is the guest of honor, all eyes are on him and he may be somewhat guarded in his body language toward you. If you meet at the park, however, the man may be able to display all of the body cues available.
Step3
Know that your body language can affect a man's body language. For instance, say there is a man interested in dating you. He is about to come up to you with a big smile on his face, showing his interest. However, you're in a bad mood, and scowl as he approaches. He may not display that same smile if he thinks you aren't interested, or if he senses it's not the right time.
Step4
Know the cues that men typically give to indicate what type of relationship they want. If the man wants a business relationship, you will get a formal firm handshake as you greet. There will be directed eye contact, usually at the end of phrases. The man will keep a distance from you, but will be facing you as you talk. A man interested in friendship will make eye contact, but of limited duration. The man will turn towards you sometimes, but not throughout the entire conversation. The man will seem interested in what you say when you talk about normal subjects like sports, and there will be some smiles, but not ear-to-ear smiles.
Step5
If the man is interested in a fling, he will get too close too soon. He will come right next to you, and may seem to invade your personal space. He will keep eye contact too long for comfort, and may touch you even though he doesn't know you. Expect him to try to stroke your face lightly or play with your hair. If the man is not respecting your boundaries, he is more likely interested in a fling. If the man is generally interested in dating you, he will turn towards you during the conversation. He will look into your eyes longer than a friend would but not as long as the fling guy. He will have big smiles that show he is generally interested in the conversation and you.
Step6
Look for signs of fidgeting to indicate If the man is not interested. The man may look away frequently, and shuffle his feet from side to side. He won't be smiling, and may have his arms and/or legs crossed. Just understand that some men are distracted easily, so if he starts watching television or a passer-by, this doesn't mean there's no interest. Remove him from the distraction and see how he responds at that point.
Step7
Read body language over time. If at first you can't guage a man's body language, wait until the next encounter. You will generally see a pattern develop of how the man reacts to you, and your accuracy in guaging his nonverbal responses will improve with every time you talk to him.
Enthusiast 
