clipped from www.myhonestmechanic.com

Serpentine Fan Belts — Cars these days usually only have one or two very large belts that drive multiple accessories...i.e. power steering pump, a/c compressor etc. These belts are very long and very strong. The rule of thumb is if you can see two or more cracks within an inch of belt space you should replace the belt. When inspecting your fan belt you should do so on the back side of a pulley where the belt is being laid back. As you can see (or not see) there are tons of cracks in this belt when inspected from the back side of a pulley but you can hardly see any visible cracks on the same belt to the left of the picture when it is not bent backwards.

Small cracks in the rubber can lead to belt failure