In my only experience with one case, I used the fingernail test.... It was on a Subaru that I had used a CV boot kit and CV grease from Advance Auto (never do!) The grease was so poor the joints starting popping under load as if there was no grease in it..

Tore it back down, bought the more expensive kit and grease from the dealer and rebuilt it with no problems.. It had light scoring that I could see, but when running my fingernail down the metal, I didn't feel any deep ridges that the joint could hang/bind on, and make the popping noises (of eventual self destruction). If there's deep grooves/ridges, it's probably toast!