Mac: We've discussed how a keen eye can help you detect if an image has been photoshopped, but if you need an app to help you determine if that photo is a fake, Photo Police is a utility for the Mac that can help. If it does detect that an image has been modified, it will show you what's been changed so you can get an idea of what the original may have looked like.
Photo Police is $1.99 in the Mac App Store. For the price you get a tool that's surprisingly good at detecting even small changes to photos and other images. The app does a great job at detecting obvious and subtle modifications, and while it's not foolproof, it's definitely good enough to help you determine whether that photo you stumbled across on the web is really a photograph or a mashup of other photos put together for effect.
Admittedly, if you have a good eye, you may not need an app like this, but it is fair to say that Photo Police takes a look at details your eye may not be able to discern. Do you have another app that does the same thing? How do you determine if a photo is a fake before calling it out? Let's hear it in the comments.