How do you tell your Cartridge needs replacing?

Microscope required!

@jasonbourne, can you recommend a cost effective microscope to do the job? Also how do I know what acceptable is in regards to the health of the stylus? Also would I have to remove the cartridge to examine? Sorry but never done this before.

Old school solution...when you get tired of hearing it and you want something better. Unless you have your dream cartridge, and you want to keep it going. Then just send it back for inspection/stylus replacement.

When sibilances  sound spitty & rough when they didn't before.  When violins that previously sounded smooth sound saw-toothed. When your favorite guitar player's guitar sounds fuzzy when it used to sound clean. When did that dude buy that fuzz-box?????? When a piano sounds rough and hashy when it didn't before. When a voicalist's voice starts to break up at places it didn't before. Yeah, I've encountered more than my share of this over the decades. On the bright side, I disagree with the posters who say that once your records are ruined once they start sounding rough. More than once, I've installed a new cartridge whose stylus managed to find a portion of the groove that wasn't hacked to death by the old, worn-out one. Good luck!

You should listen for signs of mis-tracking on the most demanding points of tracks with very sharp and loud transients, such as where a female singer gets too close to the microphone and really hits the note hard.  If you start to hear a fuzzy sound or strange noise where this never happened before, you have mis-tracking (the stylus is no longer in proper contact with the groove and is slamming the grooves instead of tracing the grooves.  When the stylus is worn or the suspension has gone bad, mis-tracking increases.  Before this point, it is hard to hear signs of wear because they happen so gradually.

I don’t think that microscopes are that useful for anything other than seeing gross wear, or major damage, that would be obvious by listening.  I’ve had “experts” examine and pronounce healthy, cartridges that sound worn or defective.  It is particularly hard to see and interpret wear on narrow contact cartridges (e.g., microline, microridge, van den hull, replicant 100, shibata) because you need high power magnification (which means poor depth of field so only a portion of the stylus is in focus), appropriate lighting, and an expert eye to interpret what is visible.

If you have doubts about a cartridge, either decide to replace it or the stylus before risking record damage or send it to the manufacturer or companies like SoundSmith or Wallytools.