Ok - time for a less smart ass answer.
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.
Enjoy - ribbons are awesome.