Any serpentine belt with more than 50,000 miles of service is a replacement candidate. Also, a minor amount of travel left on the automatic tensioner is often a signal that a belt replacement is in order. These belts are often hidden behind a guard cover. On many vehicles, there is an access hole where the belt may be viewed. If the belt shows signs of wear or missing teeth, recommend replacement.
Many engines are also interference design.
This means that when a timing belt jumps, the engine valves stop, with some halted in the open position. In an interference engine, there is no clearance machined into the piston. This means the piston kisses the valve, and mechanical damage occurs. A failed belt, therefore, can cause a major engine repair. You can easily see that, in this type engines, belt repair is a service that must occur at a recommended interval.