Topic: how to tell bad bearings

[bbb]

i cleaned my split bearings on a brown marble.  How can you tell when they should be replaced?  they have some wear where the rod slides back and forth.  when i say wear, i mean the texture is no longer smooth finish, grayish instead of black.  does that constitute a change?  or would a brand new bearing look like that after a few plays?  how do you know when they need replaced?  thanks