How do you know when tubes are done?

all posters gave good advice. IMO, the best was to keep a replacement set on hand at all times. If your tubes sound "tired" the new set should easily allow you to hear a difference.

A tube amp owner is often a tube geek to some degree, and "replacement" sets become laughable as one likely has a drawer full of the things. I could go another 10 years on my current stash. I recently broke a preamp tube (and had the power amp driver tube start showing its age) which launched me into a 6SN7GTB maelstrom of tube option messing about (mostly NOS various things moved around until the mojo returned)...hard to pinpoint when I got it all dialed in other than to say it all got right somehow eventually...changing out a rectifier (!) tube was surprisingly helpful, as I hadn’t noticed it needed changing, but there ya go...you know when tubes need changing when while listening your brow becomes furrowed instead of not so much...

I have replacement sets of Western Electric 348a tubes (my stereo amp runs four of these at a time).  But, after many years of operation, two tubes started to go weak.  Although I have replacements, these tubes are so expensive, that I have, for now, replaced them with much cheaper "equivalent" tubes.  The disparity in price is crazy--the last time I could even find a 348a offered for sale, it was being sold at $1,500 each for old, but testing "good" tubes.  The 6J7 equivalent sells for around $7.50.  Now THAT is a big difference.

Usually a browning of the getter flash indicates a weak tube, but there are a few types in which this is not the case and the tubes can still be viable. A black thick getter flash is fine. I think it was GE 6L6GCs that had a browning tendency that turned out to not indicate problems.