how do i know if my rear gears are still good??

im pretty sure i need to get a new carrier and carrier bearings, and i wanted to use the stock 3.91 gears how could i tell if they are still good? I would not want to get it all rebuilt and have the rear hum or howl. they look in good shape. any ideas??

can have them mag. tested for cracks . if gears show no wear or cracks- good to go. before removing gears from case , get a pattern (same as when setting them up) and the back lash . with that info , can set up the same

ok thanks