Strobist: Reverse Engineering Other Shooters' Light:

Over at Strobist, the lighting experts are talking about how to figure out what sort of lighting another photographer used - simply by looking at the photo.

Of course, you're never going to be 100% accurate - but you should manage to get enough of a solid idea of how to duplicate the lighting used for your own purposes.

I read somewhere, about a certain skilled painter trying to duplicate the Old masters - not to sell them as forgeries, but to learn their techniques. There's no reason that we, as photographers, can't do the same sort of thing...