I'd say just a lack of transients and response to frequencies over 1kHz. I havent worked with a massive range of ribbons, but it is pretty obvious when a $1k mic doesn't sound right.
If you have concerns about an existing mic why not post a clip with some details of the mic and we can aid you a little?
Or if you're asking generally if a ribbon mic has a maintenance cost a few years in... I'd say a ribbon will last you several years of infrequent recording, meaning it depends on the ribbon itself but at least 5 years at very minimum.
Loss of output is a bad sign - I had a mic that was tonally close to its original sound, but needed much more gain then it did to begin with - ribbon was badly bent.
Also if you have the gain up and you gently rock the ribbon back and forth any kind of "clunking" noise means that the ribbon is loose and moving back and forth in the magnet.
Depending on the mic, you can also disassemble to do a visual inspection.
Remember to always store your ribbons vertically so you dont have gravity sagging em out when not in use.
Also a good idea to keep a cover over the mic in between uses during a long session and have it covered when moving it around the studio - and tell people NOT TO BLOW INTO IT! For some reason it is like a red flag to a bull when a singer hears the distortion of ribbon from their breath - they try to make it happen more, "listen to that funny sound I'm making". Using a pop filter from the get go lessons this kind of fun.