I just read a very interesting story from the folks at Scientific American. It is titled the five ways to spot a fake photo.

Since many of you are in the business of editing photos, I thought I’d pass along the five ways along with a link to the story with all the details. The five ways listed are lighting, eyes and positions, specular highlights, clones and camera fingerprints. For each of the ways listed, the article has examples and diagrams which explain the problem and potentially ways to avoid the problem.

A related article on the site has a slide show showing photo tampering throughout history.