How Women Decide Which Men to Sleep With

Have you ever heard something like...
"A woman decides within the first 30 seconds of meeting a guy if she's going to sleep with him"
...?
I've heard a lot of things like this over the past few years from men and women alike, so I decided to try to figure out if this idea was actually true... and, even more importantly, how a woman actually comes to this important decision so quickly.
Well, the bad news is that I've never found the direct answer to this question!