I would just get kind of friendly with him. If he takes an interest in you, ask him about his size. Some average/small guys will, believe it or not, answer you honestly. I have friends who casually say they have a "medium size ship," or something to that effect. A small guy won't say he's small, but he probably will say he's "average." If the guy says he's big, then ask if you can feel it. If he says "yes" then the ball is in your court as to whether or not you feel he's big enough. If he says "no" then he likely believes he's not really that big.
One suggestion though-- I wouldn't ask him to whip it out in a public place. I have girls ask me to do that to "prove" my size when they find out I claim to be large. That's offensive to many guys (myself included), and suggests you're "calling his bluff." Many guys will say, "F*** you, I don't have to prove anything." But, asking to "feel it," is waaaaay different. That sounds like you're intrigued and curious, as opposed to confrontational and suggesting he's a liar. And it's discreet...not pressuring him to publicly expose himself.