How can you tell?

If you're grinding against a guy you most likely are going to give him a boner, either a semi-boner or a full one depending on how long you dance. But sometimes that can happen pretty fast if that's the girl's intent. lol. A guy would have to be pretty lame to not get anything unless you're barely touching him. Whenever I have a girl grinding against me on the dance floor, "something" almost always happens. Usually a girl will then just face me and like giggle to herself and we'll dance facing each other for awhile or she'll just face me and then giggle and then walk off and tell her friends and maybe some other girl will come and take her place. But you should know what you're grinding against and if that hard spot wasn't there before you surely gave him a boner. Something hard doesn't accidentally appear in his pants. lol. Depending what the guy is wearing can conceal it somewhat but you should still be able to feel it. If you thought you felt it the chances are that you gave him one big time.

If you feel something hard rubbing against you and it isn't a leg or arm, it most likely is a penis. Also, his pants might bulge a little.

It shouldn't be too difficult to tell.

If you felt it mostly likely that's what it was. Like desperado said if it creeps you out, leave to get a drink or get back to a friend. If you like the guy turn to face him, or maybe even give it a brief arm/hand sweep. You might get lucky.

Yup, I agree with Hercules, you feel something hard, It's definitely a boner. Just to let you know,usually when I feel it, I stop dancing and get something to drink, or if like him, I'll just turn around and face him dancing, he always seems to like that.