Did you watch any of the Sherlock Holmes TV series that was on BBC last year?
Basically, Sherlock looks at people and after a minute or so he knows pretty much everything there is to know about them. He analyses their clothes, their body language, their speech patterns. Now, when he does it, it seems an almost supernatural feat, but he never says anything that is beyond the realm of logic. He just studies people and make logical conclusions.
If one man can do something, that means any man can do it. To do it on a basic level just requires a bit of practice. As a little warm up thing, when I'm on the tube, I just look at people and see what I can deduce about their characters. You would be surprised at how much you can come up with just by looking at someone and thinking about what everything means. Also, with everyone you know and after every interaction you have, think to yourself "how could I have known x about them from the way they presented themselves?". You'll start to notice patterns and you'll get better at assumptions.
The brilliant thing is that it doesn't really matter if you get it right or not. Mix up assumptions with a bit of humour and it doesn't matter if you are completely off the mark.
Here's a simple example. Say you see a girl on the street on a weekday. This ALREADY narrows down the set of assumptions that you can make, she is a tourist, on a lunch break or has an evening job. If she is a tourist, she will be wearing casual clothes, not be walking with any sort of purpose and have a foreign accent if you talk to her. If she is on a lunch break, she will be wearing business clothes and be walking with purpose. Otherwise, chances are she has an evening job, is a student or has some job with irregular hours (eg. a model).