CracksCracks are a clear indication that you should replace the drive belt. This is usually a sign of old age or improper tension, both of which stretch the belt farther than comfortable. The excessive stretch slowly pulls the rubber apart, creating cracks. If there are any cracks that are more than halfway through the belt it needs to be replaced immediately—a crack can quickly turn into a tear and cause the belt to slip or snap.
GlazingGlazing on the drive belt is usually caused by aging. As the belt runs over the pulley system, it will become worn and hardened, and begin to look smooth and shiny. This is especially important to monitor once it affects the backside of the belt. If the belt can't get a proper grip on the pulleys, it can have a devastating impact on engine performance.
Read more: Tell-tale Signs That Your Engine Is Due for Serpentine Belt Maintenance