You know if a guy likes you by his body language; that is how he looks at you and see if he compliments you. You need to also notice eye contact and the way he pays attention to you to indicate his interests towards you. Find out more details at http://www.ehow.com/how_2044924_know-guy-likes-you.html.
If he shows a lot of concern for you when you arent around. Or teases you. thats how I know. my math teacher tells me my friend could like me cause he shows a lot of concern. just to check you may wanna ask him.