Ok I'm not a guy and since no guy found time to answer these questions may be I can try
A guy feels happy whenever the girl he likes is around. He would do all kinds of stupid things to extend the time or would talk her into staying a little while longer...stuff like that.
If a guy isn't interested in you, you could tell that if he looks at some other girl with interest or keep going to her. If there is no girl, then if the above-mentioned things are NOT there the guy isn't interested in you.