This is what Malcolm Gladwell calls 'thin slicing' - the brain's ability to sift through vast amounts of information very quickly, select the most useful bits, and draw roughly accurate conclusions based on those pieces of information.

And importantly, that this is all done unconsciously. You don't look for these signals, you don't seek to recognise them. Its just a sense you get.

You just know.

The ability to thin slice during our interactions with women, as with everything else, isn't something we're born with. Its something we must learn, and we will learn only by experiencing hundreds or thousands of interactions with women.

Someone who has been active in Pick Up for 10 years, approaching thousands of girls along the way, will enter an interaction and very quickly - and unconsciously - thin slice the girl's responses and make deductions about her reactions which will guide his own actions. People who haven't been in Pick Up as long - someone like me, who has only been practicing for 2 or 3 months - will be less able to do this, and will either have to do it consciously (for example, searching for IOIs) or else just accept that this ability will come with time (my preferred approach - I think IOIs are rather arbitrary).