Pay attention to how physically he is to you. The closer he is, the warmer he is thinking of you. If you move slightly closer to him, does he move slightly further away? That means he don't want your interaction to be any more personal than it already is. If he don't move further away, then he is receptive. And if he respond by getting even closer to you, he probably really like you or are very comfortable around/by you.
Watch his actions to see if he treats you differently from the rest of your group. If he's really interested in you, he may start to behave protectively toward you, or in a "gentlemanly" fashion (at least to the extent that he interprets his behavior as such). Look for signs like shifting his chair closer to yours, putting his arm around the back of your chair, leaving his jacket on the back of your chair or even going so far as to place the jacket around you to ward off your complaints of being cold.
Be aware that some guys flirt with other girls to get your attention. It gives him a chance to see your reaction, and helps him know if you really do like him or not. (Yes, it's an odd sort of logic, especially since you might end up so offended or confused that you just give up on him!) However, you can usually spot a "get-your-attention" flirt if, in the middle of his flirting scenario, he keeps taking the chance to look at you, seeking out your response. You can also try a quick trip to the bathroom and find yourself a sneaky observation point to check out how the flirting's proceeding. If he stops the moment you've left, it's you he's serious about, not her. Alternatively, ask a friend to do some observing for you while you're away.
If he hits you or playfully punches you on your arm, this may mean he likes you. A guy may gently hit or punch you on the arm as a covert, "manly" way of getting to touch you without making it too obvious what his intention is. If he finds that you don't pull away too much when he does this, he might find the courage to proceed to more gentle ways of touching you. Of course, this doesn't mean you need to sit there in pain if he actually hurts you––be assertive enough to point out that it hurt! You can salve his pride by saying something about not minding him touching you but to please watch out for your sensitive bony bits! And if you're the kind of girl who finds the play punching a bit of fun, give him a playful knock right back.