Sometimes plastic display food, like you often find in Japanese restaurants, can look awfully real. This website has a shockwave test
that challenges you to separate photos of real food from the plastic stuff. The pictures are too small to really make a good decision, but it's a neat idea. (why they felt the need to design the test in shockwave, I can't figure out. HTML would have worked perfectly well for something this simple, and when viewing it with Safari it crashed my browser).