He always wants to be around you. He talks to you more than his friends. He stares at you. He plays rough with you. He tickles you. He smiles when you catch him looking at you and gets embarrassed. He shows off when you're around.
It depends, up to like freshmen year guys will tease you or make fun of you, and maybe even appear to be avoiding you. They're just nervous. But after that, they will try to hold a conversation and...
if he starts maybe staring at you and smiling, or constantly looking at you and turning around, or possibly even asks you to hang out or go do something, signs point to YES!
If a guy is not into you often times when he talks to you he will answer in short phrases and will be looking around him (to see if he can find an excuse to stop talking to you!) Or he hardly talks...