Drive belt tensioner wear indicator

I have never heard of a wear indicator, although that does not mean there is no such thing. The majority of engines use a spring-loaded tension-er pulley. V-belts grab their pulleys on their sides and wedge into them. As they wear down, the belt sits deeper in the grooves. That makes them become loose, and periodic adjustment is required. Flat serpentine, (multi-ribbed) belts do not wear the same way, so there is less tendency to become loose or need adjustments. The common rule of thumb when assessing wear is to look at the cracks that form across the ribbed side of the belt. It is typically time to replace it when you see more than one crack per inch.