How do I know when to replace the idler pully? on 1990 Chevrolet S10

the serpentine belt came loose from the assembly but did not break. I found the doughnut shaped pully had fallen off from the bearing race. It was laying down under the radiator fan. Are there two pieces or did something break?

Asked by for the 1990 Chevrolet S10
Sounds like the tensioner just came apart and needs to be replaced. Probably a good idea to replace the belt also. Hope this helps