How do I determine if an image has been photoshopped?

Question

Does anyone know how to tell if an image has been photoshopped?

edit Specifically, I want to determine if regions or pixels in the image have been altered by photoshop. I'm not as interested in finding errors in the photoshop job (missing shadows, errors in reflections, etc)

I seem to remember something like a high-pass filter that you could use that would determine the noise levels. Edited regions would have less noise, or at least changes in noise. I'm having a hard time finding information on this or other approaches.

Thanks for your help!

Answer 1

I use this website to check if an image is photoshopped or edited: http://imageedited.com