[Walter White]

I have been doing some research on ball bearing failure, and there is one test you can perform with a bearing in place.
Remove the seal and check how loose the cage is (also called a separator or retainer). On OEM single row bearings, check that the cage is not able to touch either the inner race or outer race. If it can, a bearing failure may be close.
I haven't done as much research on double row bearings, but on OEM double row bearings, the cage is a bit different. It is easy to see if it can touch the inner race, but you have to look very closely at the cage to see if any part of it can touch the outer race.
These checks should be performed on new bearings before installation. On single row bearings, you want to check that the cage cannot be pushed more in one direction than any other direction. In other words, it should remain fairly well centered.