Digital cameras store a lot of information about the technical specs of the picture called EXIF Data. The camera stores info such as times/dates/fstop/lens/shutter speed/ISO/mode/shutter count/ and many other settings. Programs like Picasa Albums and Flickr make this EXIF Data available to the viewer (unless the user somehow disables or doesn't upload it intentionally). Let's try it. I have this flickr image from my favorites
In the Actions drop down, you can go to View Exif Info
In this truncated view, we can see that it was taken at ISO 50, 116mm, f18 at 0.6 of a second with a Canon DSLR. It was shot in manual exposure (out of view on this screenshot) and the on-camera flash did not fire.
From Google's Picasa Albums, we see this from a different image:
This shot was take with Nikon D70s, at ISO 400 etc etc
There are a few free programs out there that allow you to look at the EXIF data of your own images. You may already have a program that allows you to view that information.
Chase Jarvis sometimes does a little image breakdown where he shows an image and people guess how it was taken. Some of his lighting techniques go right over my head, but there's some good stuff in there.
ONE FINAL TIDBIT:
Some people include their lighting notes in the caption of their images on Flickr. They generally begin with the word "Strobist" and as in this example.
Pic of the Day: Oregon coast shot with a Graduated Neutral Density filter.