Look at how he treats the person you think he might have a crush on compared to normal people comparable to her. Like is she's a close friend of his does he treat her differently from other close friends, or if he doesn't know her at all does he look at her a lot.
There are many different ways, depends on the guy, but like nobodygirl said he'll treat that someone special just a little bit differently as either if he is a nice guy he may not want to "rub her the wrong way" or if he is outgoing he might tease her and push her buttons a little bit.
But he will find a way to be around his crush, maybe not so close but he like's to see the girl that he has his eye on, she what changes on a timely basis. May not everyday whatever, but he'll make the best of his opportunity to be around his crush.
Well some guys can try to show off in front of you so they could get your attention. Or some could be the shy type and not always knowing what to say when you two talk. But the most I have seen is that a guy stares straight to your eyes when talking. Or he could still be staring at you but you not knowing