I've looked at styli with special stereoscopes designed for the task. With my untrained eye, I could not really see wear that my ears clearly said was there. The dealer with the scope also pronounced healthy cartridges that I thought were clearly worn (or had other problems like the elastomer in the suspension going bad, etc.).
The short of this is that one's ears are clearly superior to any visual inspection short of something exotic (like a scanning electron microscope).
The short of this is that one's ears are clearly superior to any visual inspection short of something exotic (like a scanning electron microscope).