Fortunately, the motors and signal processing components usually outlast the gear trains and potentiometers. Gears are easy enough to test for, and pots show up as a case of the jitters at some point in the servo's travel. Testing like these guys are talking about will ferret out the servos that are defective from the factory, and from there it's highly unlikely you'll get a failure other than gears or pots for years to come.