I sort of like str82fcuk's approach.
I would wait until I am alone with the guy and make a casual comment like, "damn those pants make your crotch look huge - you are really that big?"
In all events, he is clearly cock-proud (I would know). And he isn't trying to hide it, so he will appreciate the compliment.
If he is straight, he will say "yep".
If he is gay and interested, he will likely know that you are maybe interested to see more.
It is a spark - if he feels froggy, he will be more inclined to jump.