Here's a jpeg capture
of Serious Sam:SE rendered by a Radeon 8500LE. I suppose you can use an image taken from a camera to look for certain types of cheats like a missing reflection but obviously you really cannot judge AA, AF, and other details by the pixel level with these kind of captures, and I'm afraid there will be a plenty of idiots making claims that NVIDIA or ATI has better image quality because of a camera screenshot.
Things get worst when the captured white balance is not correct or when the highlights have been overexposed or when the image is noisy or when ... well you get the idea. :P