yo i tried this line at least four times at the bar last night and i got at least a dance out of it if not more...it worked for me and thought you guys might want to try it out:

at a club and you see a group of girls dancing with each other (classic girls dancing in a group) preferably only two or three girls so they don't out-number you horribly.... anyways, go up to the one you'd like to dance with and say:
"hey! you look like one of those girls who only dance with their girlfriends and have like totally never danced with a guy, is this true?" my responses i got every time was: "what? no way man" or even better, "wanna find out?" either way i say: "i don't believe you, you gotta prove it to me, lets go see what you got"

this way, the girl is going to try extra hard while dancing with you and you have the power to tell her if she's good or not...its win win!

i like it... but it leaves a window for her to say "yeah, now go away" if she doesnt find you physically attractive or if she's in a bad mood, it's perfect if you've gotten an approach invitation from her though.