Hi Benn,
Telescope mirrors, apart from the front-surface reflective coating don't really deteriorate with age provided they are cared for and occasionally cleaned carefully.
So far as assessing a mirror in a second hand telescope, you probably need to become familiar with star testing the optics and examining at high-power the intra and extra focal star images. These will reveal nearly everything you need to know about the figure on the mirror. Perform a search using star testing telescopes for a wealth of information.
The most common errors found in mass-produced mirrors are under correction (spherical aberration) turned edges and surface roughness. Star testing will easily reveal major defects. You need to carefully and correctly collimate the 'scope prior to star-testing or the results will be misleading.
An alternative that is also helpful is to make or buy a Ronchi eyepiece like this:
https://www.teleskop-express.de/shop...th-a-star.html
or
https://agenaastro.com/jsp-easyteste...ng-tester.html
that will give a very good indicator of any serious (and subtle) problems (following careful collimation).
Of course one other way is to remove the mirror and perform a Foucault (knife-edge) test. You will probably find most people selling a 'scope will be reluctant to disassemble it and remove the mirror for testing. Best to do it in situ with a star test or Ronchi eyepiece. I have owned an easy-tester for about 25 years and it is the "most borrowed" piece of equipment I own.
Best,
L.
P.S: BTW, if you enjoy being really, really unpopular with others, take your Ronchi eyepiece to a major star party, ask people if you can look through their telescope with "my own eyepiece", insert the Ronchi and then tell them bluntly and candidly what's wrong with their telescope. I haven't done this myself but have witnessed it. Nearly caused fisty-cuffs.
Sure fire way to end up with a bloody nose (actual or metaphorical) within 30 minutes.