How can you tell when tubes are shot?

You may not notice the slow decline. You may be use to the sound. Since you will need new tube eventually, get some and see if it sounds better. If the same, just save the new tubes until later.

What Sugarbrie said. The sonic decline happens so slowly,
that we do get used to the sound..if new tubes are inserted,
and allowed to warm-up..there should be an obvious "ah-ha"
experience..ie: the highs are back.,.. If not, the old
ones are still pretty good.

Is there such a thing as a tube tester? Thanks for the advice, I hadn't thought that a complete failure could cause damage. Duh. We don't want that so I will pry my wallet open and spring for new ones. I just hate to be wasteful. In my opinion the system has sounded better than ever. Plenty of highs, lows and everything in between.

I was advised to look between the pins and if there were dark spots between them then it was time to change out. I don't know how valid this info is, but I did have some Bugle Boys that sounded soft. I pulled them out turned them over and lo and behold dark spots between the pins. Go figure. Best of luck on your search.

When I was in the Air Force, I worked on a few systems on
B-52's that had tubes..and we had tube testors..but they
don't tell the whole story. A Ferrari and Porsche could
look almost identical on paper..or in Road & Track test
results...but still "feel" and drive very different from
a driver's perspective.... and an old tube could still look
to be good, when indeed it would sound poor, or two of
the same model tube from two different makers could both
measure the same..but sound very different...IE: trust your
ears.