Notice if he is always surrounded by a lot of women, or if women seem to gravitate toward him. Observe how he responds to them, or to you. Is he touchy-feely? Try to notice subtle little touches that might seem a little more than friendly. For some men, flirting is a way of life, so it seems almost natural in their mannerisms.
Pay attention to his conversation. Notice if he seems to use a lot of flattery. Are his conversations pretty shallow? Does he seem to use a lot of typical “smooth” lines, or worse than that, does he have better ones that may be harder to resist? If he is paying too many compliments to be sincere, or approaching you like you are the most special woman in the world when he doesn’t even know you that well, he more than likely is flirting.
Be aware of “the stare.” If he likes to make direct eye contact and let his stare linger, chances are he is flirting with you. Some guys will do little things like smile or lick their lips at the same time, so there will be no doubt in your mind that he is flirting with you.
Watch how his friends behave. You can’t always judge a person by the company they keep, but if his friends seem to be very flirtatious, chances are he is too.
Take note of the disrespectful flirt. This guy will not try to hide or be subtle or slick about the fact that he is making advances at you, and his lines may be less than smooth. Often he will make a comment about your body parts, for example, telling you that you have nice tits, or a big butt. Or he may just make reference to sexual activities.