A part of the photography learning process is to look at pictures and try to reverse engineer them. I want to learn how to reverse engineer portrait images. Reverse engineering the light in portrait photos is covered here, so this question is trying to pick other aspects like the following:
Reverse "engineering" (shouldn't it be 'reverse artist-ing'?) light, whether from a portrait or otherwise is generally not rocket science, for determining whether flash was used and whether it was off- or on-camera, at least. Aperture and shutter speed are tougher to know without knowing additional variables, but those two items are actually not that critical, anyway. They are important when you shoot, of course, but it's not important to know them when you're trying to figure out what another photographer did...the main thing is depth-of-field, and especially with a portrait, you can get a pretty good guess on that by just looking at the subject.
David Hobby (Strobist) has a couple of really good blog posts on "reverse engineering":
You may want to look over his whole Lighting 101 series for a great tutorial on lighting.
