personally I will make strong eye contact with most girls at my college that I am interested in.
For example, when you both look at each other and one looks away. I trained myself to not look away first. Apparently, it makes me more confident. I personally love it when a girl smiles first. This gives me that hint to a least talk to me. Another time is when a girl looks at you and then looks down.
If he smiles back, that's a good thing. Try to get his attention by smiling or talking.
It means you're either really hot, or just the hottest girl in the room. Girls are so complicated and stuff though, so he's probably thinking there's no point in trying to hit on you because you're just another dumb, complicated girl. That's usually what goes through my head. I hate trying to get girls and play games. So anyways, he's probably just checking you out once in a while because you're some decent eye candy.