these are some things a nice guy, like me, would do.
a major sign would be anything that involves contact with each other. so playful poke, nudge, punch, or rub. this is typically on the shoulder, back, or arm area of your body. if it's done constantly, chances are the guy likes you, unless he is know to do this a lot. but other than that, it's a safe bet.
second would be looking/staring at you. if you see him taking a glance at your direction, a light bulb should light up in your head.
lastly, if a guy tries to find a way to get you alone with him. so a study session at school, lunch, snack break, a drink out, etc. it's a clear indication he wants to talk to you and get to know you a lot better.
also, there are shy and proactive guys. coming from someone who is shy, but trying to break out of that shell, i typically do the 1st and 2nd signs first. if they get reciprocated, i move onto the 3rd sign. as for the proactive guy, they usually start with the 3rd sign right away.
hope that helps