How do you know when tubes are done?

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."

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.