Look for him watching you or looking at you, and, if you can, see HOW he looks at you; Does he seem interested in you? Like he likes looking at you?
If you are friends or in common social contact somehow, then he might try and sit/stand/be closer to you. Obviously, be on the lookout for things like smiles or unrequested compliments, and see if he does things just to be near you, like walking the long way home or waiting/taking a long time, when he doesn't have to.
It really depends on the situation (ie. someone you attend school with, a random at the mall, someone you work with, etc). Lets assume its a guy at school. Depending on his personality and level of comfortability with girls, he may approach you and speak to you or he may just sort of glance at you and look away or he may stare. Try being a little more specific and you might get more accurate info. :)