Run the engine and watch the belt(s) and pulleys. If a ribbed belt seems to be flexing fore-aft, stop the engine and see if it's properly installed on each pulley (it'll track if it's one groove off, but transfer power poorly and expire prematurely). Watch each pulley to see if it wobbles, which indicates looseness or misalignments. This also affects power transfer, which may cause a belt to pop off and can also damage the shaft bearing of the pulley's accessory.

Misalignment often results from a stackup of manufacturing tolerances and shimming a pulley or the accessory's mounting bolts may be needed to correct it.

Most late-model cars have a single ribbed belt with a spring-loaded adjuster, and although you can't change the belt tension, you can check it. Most GM cars have a tension indicator on the spring housing, and as long as it's not at the far end of the slot, the belt has not stretched excessively. If there's no indicator and there's a specification for belt tension in pounds, use only a professional-quality gauge made for ribbed belts (and calibrated in pounds of strand tension) to check. The ribbed-belt gauge also can be used for ribbed belts controlled by an adjuster, but you'll need a different gauge for a conventional V-belt. In either case, the adjuster typically is a jackscrew. Loosen a locknut or nut, then turn the jackscrew to increase or reduce tension. Some car manufacturers provide a belt deflection specification instead of tension but it's more difficult to use this accurately.