There are methods to determine if there have been modifications done to an image in Photoshop, but quite a few rely on having other supporting images. Most of the detection process deal with examining the pixels/grain at a high magnification.

With technology advancing as it is, it becomes easier to do and harder to detect (for those with the time and talent), but there isn't a "litmus" test.

There are some good articles about this subject on the web, though they deal more with the issues of Ethics of Photo Manipulation and Journalism vs. Marketing

Good subject to think about...

:)
retrocity