There are several signs a pulley is no longer functioning properly. First, check for free rock. There should be no movement or play from side to side. Next, look for misalignment. It will cause a belt chirp noise and is often the result of worn bearings. Spin the pulley with the belt removed, if it spins more than one revolution, the bearing grease is depleted and the pulley should be replaced. Roll the pulley by hand to feel for rough spots during rotation. You’ll also want to check for physical damage like cracked or broken pulleys, discoloration, surface build-up or excessive wear. Finally, a pulley with seized bearings is also a sign replacement is needed.