Obviously if a bolt has stretched or is corroded, bent, or visually damaged, by all means throw it away. If you are not sure, you can measure the hardness, check its structure and grade (destructive tests, in general) and chemistry to see where you are. And as far as fatigue one must know if the stress on it is in the fatigue range, and so is temperature, that has to be high enough to affect the metal. All of these tests make sense in a lab, but practically it's much cheaper and easier to use new fasteners, so I don't have a problem with that.