How can you tell if a stripper will let you fuck her for money?

The one easy way, at least within my experience, is if she asks you for a ride to her home, hotel, etc. You don't think she wants a ride just to have someone drive her home now, do you? Sometimes, they ask if they can do a "really private" dance, or words to that effect, at least.

The key here is trust. Ladies who do this sort of thing obviosly want the money so, if they feel they can trust that you are not a cop or a psycho, they will come to you. The WORST thing you can do is walk around a club asking...

Ask if she is full service.

Usually, she should punch you out for asking. If she doesn't clobber you or send for the bouncer, you just might be in luck.