Flirt.... a lot
Forgive me for answering even though I am a girl. I find it isn't hard for me to tell if a guy is interested. Just see if you can look into his eyes for more then 3 seconds. Then just look away.
Every now and again, look him in the eyes, smile, and hold his gaze for a moment. Do it when he says something funny or compliments you. If he's into you you'll probably find he starts doing those things a lot more... it's a Pavlovian kinda thing :P