i would like to refer you to a book called blink which gives in vivid detail how we bring our entire system into operation to make judgement within seconds.our entire system picks up all the clues verbal, non verbal and analyses them aginst data bases stored since we have been alive and comes up with a snap judegement which most often is true.
the only hassle with it is that we are constrained by our own experiences and if we have a bad experience with one type of person we extrapolate that on to all others.
BUT yet 99 per cent of the times we are right.we have to though make sure we are not being misled by our needs of the moment. for instance for anyone lonely it is easy to see others through rose tinted glasses and fall in love with a wrong kind of person.
but by and large yes we can judge people fast and most mistakes occur only because we overlook the advice our system is giving us.