I would say you can likely tell when they should be replaced by the look and feel of the the bearing after you have washed it out with varsol or something similar. We just did this on a boat trailer that we acquired this spring. My younger son and I decided to clean and repack the bearings before hauling the boat on the highway. I was not happy with the bearings. They appeared to be looser than I thought they should and had some pitting and discoloration on the rollers. A quick trip to the local autoparts store yielded some new ones and we installed them. The races came out relatively easy with a brass punch. Putting them back in was more difficult as the narrow side of the race was just damaging the brass punch once it got even with the housing. I was looking around the garage for a socket or piece of pipe the right size or something to use to bang them in the rest of the way when I found a nice set of race drivers that I had bought and forgotten that I had. The right tools sure make a job easier.
Randy
