The 10 minute exam.
It may take you a little longer at first, but you should know within 10 minutes if you are going to get her phone number or get rejected. You may think 10 minutes seems a little long and that you could do this in a two or three minutes tops, but there are many factors you need to take into the equation. Does she have a boyfriend? Is she married? Is she in the right place mentally? What level of interest does she have in you? Once you get good at steering the conversations in these directions, you should be able to find out everything you need to know within 10 minutes.