Scientific American has a captivating article detailing 5 ways to spot a picture that has been doctored. Take a look at the photo below. If you study the light dots on the irises (specular highlights) on the four stars, you will see that two of them (Randy & Paula) are the same - which means that they were actually in the same place when the photo was taken. Simon & Ryan have completely different specular highlights. To me both these guys look pasted in - I think the lighting on Simon is all wrong too.
Surrounding lights reflect in eyes to form small white dots called specular highlights. The shape, color and location of these highlights tell us quite a bit about the lighting. In 2006 a photo editor contacted me about a picture of American Idol stars that was scheduled for publication in his magazine (above). The specular highlights were quite different.
The highlight position indicates where the light source is located left. As the direction to the light source moves from left to right, so do the specular highlights. The highlights in the American Idol picture are so inconsistent that visual inspection is enough to infer the photograph has been doctored. (Link) via Boing Boing