[Easy Ten Points] Quick Question?

How can you tell if a guy likes you? There's this guy at my high school who, I've been told, flirts with me. Obviously, this is a good sign but how do I know if he doesn't do this with alot of other girls? I'm a senior and he's a sopre so he might be intimidated to ask me out so how do I let him know that I am interested? Apparently most guys won't ask out girls unless they're absolutely certain about the girl likin them back.