Definitely pay attention when you're in the same room as him to see if he keeps glancing your way. If you start to notice that he stares at you on a daily basis, and if you don't have anything weird stuck in your teeth, this could be a sign that he is into you. If you catch him looking at you and he quickly looks away, that means he was definitely looking at you and not just in your general area. If you want to flirt with him, try smiling and giving him a quick wave the next time you see him staring in your direction. This is a step above just catching him staring at you. If he makes direct eye contact with you when you pass in the hallway and gives you a smile, he is definitely noticing you and interested in being friendly. In this type of situation, think about what kind of guy he is in general. Do you notice him flirting with lots of people? Or is he generally shy? If he is a flirt, he may be interested in you, but pay attention to his actions to find out for sure! Whether he is talking directly to you or to a group of his friends, look at him to see if he is checking out your response to his stories or jokes. If he glances your way to see how you're responding, this means he cares that you notice him. If he tries to make you laugh and seems happy when you do, that's another sign he may like you. This won't be true with all guys, but it's a fairly common thing that happens. If he likes you, he may get tongue-tied, trip, knock things over, or just seem a bit clumsier than usual when he gets near you. If he drops his books or knocks something off if his desk when you walk by, he may like you. Think about how you act when you get close to your crush. Chances are, this guy experiences really similar feelings when he's around someone (maybe you!) that he likes. Physical touch is a huge indicator of attraction. Does he tap your shoulder or place his hand on your arm to get your attention? If you're talking, does he reach over and make physical contact? Does he try to sit beside you in class or on the bus?

Summary:
Pay attention to how often he looks at you in class or in the hallway. See if he smiles at you and makes eye contact when you're nearby. Wait to see if he watches your reaction when he makes jokes in class. Keep an eye out for tripping or other clumsy behavior. Observe if he touches your arm in class to get your attention.