Checking proper tension isn't complicated. Some swear by the twist test (belt should rotate about 90 degrees by hand) while others use belt tension gauges.

If you don't have a tension gauge, just measure the deflection of the belt in the span area shown in the illustration. When properly tightened, the belt should deflect .410 to .450 inch. "Factory ...

Spring-loaded automatic tensioners are the most common today, introduced in the mid-1980s. They use a coil spring to apply constant tension and usually self-correct for minor belt stretch over time.