Well, there is of course much more than I know, but I can give you a few examples. It is mostly inferences you can draw when looking at them.
The kind of shoes they wear, the usual wear and tear of the model, as well as how clean they are kept could indicate profession (well, not the precisely profession, but it can give a general idea of the kind of work), sentimentality towards a certain pair, or a general cleanliness about them.
Similarly the hands can tell you if they are in any way creatively active (calloused fingertips usually indicate a musical instrument with strings, the most likely choice being a guitar), whereas small scars on the knuckles point towards a person that is or has often been involved in physical altercations. However, if they are unmarked, soft, and manicured (you may also distinguish professional manicures from a homemade buffing), this can tell you that a person is rarely working with their hands, but still is dependent on making a flawless impression, so perhaps a business man.
These are just a few examples that come to mind. Of course, you can always go a few steps further and draw more inferences from clothing, haircut, facial features, bearing, accessories, etc.
You have to be careful with these things, as only the most complete picture may put the details into the right perspective.