How do you tell your Cartridge needs replacing?

@ricmci , By the time you know your cartridge needs replacement you are damaging records. I use a medical microscope and special lighting to examine the stylus after 2 years about every 6 months. Most can not do this so, the next best solution is to have two functioning cartridges and after two or three years send the cartridge into Wally Tools for examination using the other cartridge in the mean time.  You can use a stylus timer and arbitrarily send the cartridge in for retipping every 1000 hours. I think that is a PITA. 

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!