Well. If the guy is interested he will ask you out. If you ask him out you might be walking into a letdown. If you really want to try though. Just walk up to him and ask him if he would want to go to dinner together. Since you asked, it would imply that you are paying, so be ready. Guys like forward women and don't like to beat around the bush to find out if a girl likes them. Even with that said, if a guy likes a girl. He will try to at least get up the nerve to spend time with her.