Stylus wear or deterioration of the rubber suspension parts will be most evident from tracking problems. If you hear the music breaking up or sounding sibilant where the sound was clean before, some kind of wear or deterioration may be manifesting itself. A test record with music passages with increasingly higher modulation levels (loudness) are helpful, assuming you can recall how well the cartridge performed on these tests when new. Loud passages, with a lot of high frequency content, particularly toward the inner grooves of the record are the most difficult to track and should show up incipient problems first.
Most microscopic examinations can only detect really gross damage/wear. I've had cartridges examined that were obviously going bad that showed absolutely no wear even when examined under the proper kinds of stereoscopes. The only pictures I've seen that clearly showed subtle wear were those made by scanning electron microscopes and no shop would have access to that kind of gear.
I would go with what I hear to make the call as to when the stylus needs to be replaced.
Most microscopic examinations can only detect really gross damage/wear. I've had cartridges examined that were obviously going bad that showed absolutely no wear even when examined under the proper kinds of stereoscopes. The only pictures I've seen that clearly showed subtle wear were those made by scanning electron microscopes and no shop would have access to that kind of gear.
I would go with what I hear to make the call as to when the stylus needs to be replaced.