If someone likes you they will probably be interested in you and ask you questions about yourself. Most people can tell whether someone likes them or not from the age of around 4 or 5
IF you truly think someone likes you, there's a good chance they have at least something going for you. I think the best approach is to test the waters, and suggest you like this person back, see how they react.
It's either he treats you special or he would avoid and you can't think of a reason why he is doing so.
All people have different signs to show they like one person. some of them act different like clumsy with you, they play with you and also flirt. Some others just don't want to see you and they admire you from a place you can't see him or her, so they love you in secret.
If they return you're smile, also if its a girl and she bends over and shows you her butt while you are watching, also if she scratches her crotch in front of you. Yeah, body language.