How do you tell your Cartridge needs replacing?

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.

I don’t think most folks are equipped to evaluate the condition of a stylus, regardless of the type of microscope they have. (I have a bunch of usb ones, and the vintage Shure stylus scope, which is cool in its own right as a period artifact). Me, I can get a lot of mileage from a good cartridge if I am careful, I inspect the cartridge almost after every side, try to avoid any sort of damage in cleaning (largely dry brushing at this point) and am obviously sensitive to the performance of the thing: if it mistracks, distorts, or is audibly obvious misperforming, you may be beyond the point where it needs to be replaced or ’fixed.’ Are your records damaged? Probably not, but don’t push it. Address the cartridge if there is any question.

I think the biggest fallacy for audiophiles is the gradual reduction in sound quality that you get used to, so a fresh iteration is a revelation. I’ve seen this a few times.

I liken this to a fresh pair of sneakers. (Get what I mean?) Vinyl is not for the faint of heart. You gotta stay on it.

Get a bottle of Stylast and use it before playing a side. It is a lubricant that is claimed to reduce wear and extend stylus life. I use it! Michael Fremer claims that the stylus will outlast the suspension!

https://www.gcaudio.com/tips-tricks/how-long-will-my-stylus-last/

https://www.sound-smith.com/articles/stylus-shape-information

We believe that styli should be checked carefully by microscopic exam by someone who knows what to look for at 1000 hours, as that is the typical maximum life for a properly aligned stylus

OP,

keep a log of how many record sides you play over a month or so and assume each one is 20 min or .33 hours.  

then estimate the accumulated hours on your cartridge based on your record playing habits.  

If it is upwards of 1500 hrs or so have it inspected by a qualified dealer.  

I had a cartridge inspected at 1200 hours and it had a lot of life left according to the dealer.