In my opinion, if a guy smiles and stares at you, that is the number one sign that he likes you. If a guy is interested or likes a girl he usually starts by smiling and staring at her, then if you smile back, he takes the next step. You may be surprised if the next thing he will do is to ask you out.
Some guys are shy. He could be smiling because it's almost time to leave or laughing at something random. But if he is really looking at you; if you are friends Then Friends; random it could be just that