Let's cut it short: There is none. Women who would say yes to such a question, or give any other form of positive reply, are so rare that you shouldn't even try to ask one you don't know in public, unless you want to make a moron out of yourself in front of other people. There is just the good, ol-fashioned flirting. At some point you will find out whether she would engage in casual sex or not, and at that point you should get ready to ask your question.

However, beware: Not every woman who looks hot to you is ready to engage in such an act. And even me, to whom I would refer to as a sexually liberal character, would feel awkward having sex after such a short prelude. It just seems funny to me.