When you strip it you see any damage. The point where they run the most will have a shine to it, that ok but if they have a groove worn in then i'd replace em. The balls and recess where they run must not be pitted either.
On rebuild i tend to build em up clean and dry then fill the cv...er backwards/inside. You want to get the greae to the back(where you cant see) first then rotate the cage and grease each ball then do the front. With most repair kits you'll get a sachay(sp) of grease, i use one full sachey per cv.