How can you tell when tubes are shot?

It depends on the tube and whether or not you are running both the plate and filament voltage. Most units have a standby, that keeps the tube warm, but not on. This can extend the tubes life quite a bit (assuming it's in standby 24/7 and on much less). It also depends on the tube, many NOS tubes can last much longer than current tubes. Mullards are noted for their long life (although I've found them to be a bit "tubey" sounding). Siemens are also long lasting and I really like them--very transparent sounding. If I remember correctly ARC charges a real premium for you to buy tubes from them, so I would recommend a different source. Upscale Audio, Tube world are two I've used with good results.

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.