Looking at his pants or facial expression can give it off. Usually a man say's he likes you. A boy in school although would not, he would probably wait till you say you like him and play games like that.
To know when a man likes you, notice how many times he looks at you, if he is looking at you repeatedly, that shows some interest from his part. Also, if the guy remembers little details of conversations you have had. Check more details on http://www.ehow.com/how_2044924_know-guy-likes-you.html.