Is your cartridge toast...maybe yes, maybe no??

I would like to question how we can determine the condition of our cartridges? Clearly if we are hearing significant loss of information or other damage on the LP being played, then that is probably an excellent indicator. However, many of us are probably using cartridges that are way past their prime, and yet they are still sounding just fine. Looking under a microscope wouldn’t seem to have much value, why...because how many of us know what the stylus is supposed to really look like...when new, when slightly worn, or when totally worn out! Can anyone really tell you what the true exact shape of a slightly worn out Shibata or Micro ridge or elliptical looks like? A completely worn out Micro ridge looks like...what? A brand new but damaged Shibata looks like what? I’m not so sure it really is possible to tell this with exactitude.
What about if the mechanicals are worn out, but the stylus itself is perfectly fine? How do we determine this factor...perhaps the cartridge will still sound good in our system, but the suspension or other mechanical parts are worn out!
I have posted before about buying used cartridges, and how IMO they are a pure gamble, but the real question now is you cartridge toast, or is it just fine and your dealer wants to make another sale??
sounds great, but dealer says it is totally worn out...risk your records or buy another or used??
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Not much ever goes wrong with a cartridge - they will last a very long time so I wouldn't worry about the cartridge body.  If the cartridge has a user replaceable stylus, it is the stylus that wears - not the cartridge.   Typical life of a MM stylus is around 1000-1200 hours if taken care of and played on good vinyl.  And yes you can see the wearing of the diamond using a good loupe or microscope to inspect it.  But you will usually hear deterioration in its sound quality first as the tip off.  Same lifespan for a non replaceable MC cartridge. 

The best way is to know when its nearing its end of life is to simply keep track of hours - round up to an hour an LP, figure how many you listen to in a week and do the math.  But you have to start with a brand stylus.

For this reason, I never buy used cartridges.  The history of the stylus is often unknown.  I agree with you that this is a big gamble.
Playing a test record every couple of months is also a good practice (I use Hi-Fi News).  It's a good reference point as to the condition of the cartridge and the alignment over time.  Generally I thought those records were useless beyond an initial setup (mainly bias and azimuth) but they really are a good indication of how your setup either settles in or degrades over time.

Agree with the posts above.  You would be surprised how long it takes to put 1,000 hours on a cartridge.  You should track it by album side just so you have a general idea.   
I use cheap "click" counters to track how many sides each cartridge has played. Very handy.
Hate to be contrary here, but I think just taking track of hours used is no indicator of wear at all...none! Some cartridges can be damaged or worn with very few hours on the clock...depending on how clean the LP is and what shape the stylus is to begin with! Others can have very long lives with no visible wear at all....visible to the naked eye that is.
Playing a test record is also no indicator....what kind of aural memory do you guys think we have, LOL!
Paraneer, do you have any idea as to what the diamond shape is supposed to look like...exactly look like, under the microscope or even worse...the loop. I know I don’t! By the time something has failed with the diamond shape, you have done immeasurable damage to your records.....hmmm. Do you hear this damage onset on the first play, the next play or the ??? question.

This reminds me of auto engine oil......  the best thing is to change the oil after every drive, and the cartridge once a week.