5- Give them the 15-minute test
It may take you longer at first, but eventually you should be able to tell in about that time whether a woman is a) interested in you, and b) up for casual sex. Reading body language will help answer part a, and the degree of her buying signals plus other information you exchange will give you an answer to b.
You might think that you could be writing off potential babes by judging too quickly. But bear in mind that it takes a certain combination of factors — interest in you, right mind-set, no boyfriend (or no fidelity) — to find a woman to suit your needs. Once you start getting good at steering your struck-up conversations through these various topics without being too blunt, it should take about 15 minutes to get a good impression of your chances with the woman in front of you either now or on your next date.
But how, specifically, will you know? Only practice will tell.