just strike up some conversation and then like if you see a couple just say aw I wish I had someone to hold hands with or kiss or something and then maybe say something like but who'd want someone like me, or something idk... and then watch his reaction and what he says to find out what he thinks of you. And if he says something sweet like ur a great girl anyone would be lucky to have you as their girlfriend then be like I wish all guys were as sweet as u, smile and stare into his eyes and stuff... I know it sounds cheesy but you never know it might work...
why don't you do it? I mean come on does it always have to be up to the guys to do stuff first? maybe he's a little shy to express his feelings to you because he's not sure about the way you feel about him.
If he already has a girlfriend, you will have to ask him out because that is the only way you will get your crush. You can either let him stay with his girlfriend or make the move and you will be his girlfriend if he likes you. Thing is he probably is comfortable having a girlfriend and would not risk the chance of dating you because of fear of rejection and losing his girlfriend at the same time if she finds out.