Assess whether he is preening.
Is he fixing his hair? Is he straightening his tie? Men are just as concerned with their appearance as women are and when confronted with a woman he finds attractive, a man will want to look his best. If you see a man straightening up before approaching you or primping himself while in your presence, it is a sign that he wants to make a good impression and wants you to be attracted to him.
Follow his eyes.
A man's eyes, in several ways, can indicate that he is flirting with you. First, men who look into a woman's eyes and hold the gaze slightly longer than normal tend to be interested in the woman. Also, an interested man's eyes are likely to check out the woman's various features - lips, breasts, legs - before bringing his eyes up to her attention again. Finally, if a man is attracted to a woman, his eyes will dilate (sometimes up to four times their actual size) while he is looking at her. The saying "the eyes tell all" certainly rings true when trying to figure out if a man is flirting with you; if he is smiling too, you can rest assured that he likes what he sees.
Pay attention to who is controlling the situation.
Men flirt by showing signs of dominating the environment. A man can be the alpha male in a number of ways. He can be the leader in the conversation by asking questions or raising different subjects for discussion. A man can show his dominance by ordering for you or by leading you into another room. Regardless of how he chooses to assert his dominance, you will know he is trying to impress you with his in-charge skills when he is focused on taking care of everything around you so that you can sit back and enjoy your interaction with him.