How do you know when tubes are done?

Oh, how do you know? No two are quite the same. One can suddenly burn out in a blaze of glory. Or die when you're not looking. If they don't do that, old worn tubes tend to lose dynamics and slam and sparkle. But it can be so slow and gradual its hard to notice. Or fairly sudden. Sometimes they will start making a kind of static noise like fabric pulled across a microphone. fffffttt.  fffffft.  Then nothing. By that I don't mean nothing it dies I mean nothing the noise stops. Its intermittent. Which can happen with any tube at any time even brand new. If it starts happening more with tubes that weren't like that before that would get me thinking maybe they're getting a little long in the tooth.

That's why I like to keep a spare set on hand. Any time I get to wondering in goes the new set. If they sound about the same that means the old ones are fine, and the new ones go back in the box for later.

I agree that the best approach is to keep a new, or tested NOS, set of tubes to put into the amp for comparison purposes.  At some point, the tubes have to be changed, so it makes sense to buy the set now.  You could get "lucky" if the set you buy now goes way up in price by the time a new set is needed.

By the way, tubes are never "done."  They can always be sold on ebay "as is;" if they you have a set of totally dead tubes, you can sell them as a "matched pair."

Tubes are finished when the silver getter flash turns brown. I ran a set of four 7591's in a Heathkit AA100 integrated amp for years until this happened. Background noise increased under the music. I ended up discarding that set of output tubes and bought a brand new set of EH 7591's. Pristine clarity restored!

Having a good tube tester on hand is recommended! That way cathode emissions can be checked before the getter flash goes increasingly brown. When emissions fall below 50% that is time to replace tubes!

Speakers were the Quad 57's. The AA100 was purchased from the original owner/builder (who built it in 1963!). So the 7591 output tubes already had plenty of use when I got it ($25!). He even gave me the construction manual! After I put some more use on it, the silver getter flashes turned increasingly browner - and noise increased, too!