Bearing clunk - but which bearing?

I dont consider my Vcore 3 to be very old, but I do wonder why sealed bearings fail, and how do we find which one? If I push my toolhead diagonally, I can confirm the clunk is not from the linear rails (phew), since it only occurs in one diagonal, but I wonder if someone has a method to identify which bearing of that particular belt loop is the problem - I prefer not to dismantle it all. And while I'm asking, why do these bearing fail at all - shouldn't they last for ages ( my belts are not overtight)