how do i know when my tubes are going bad?

i have a fisher x-202-b with 7591 output tubes. i had the thing gone over last december and have been listening to it regularly for about a year now. lately the past couple times i have listened to it it just didn't have the same sparkle. i don't know how to put it but the past few months have been great. there are times when i listen to my system and just sit there in awe of how good tubes can sound. but anyways the past couple times ive listend to it it sounds good the music is all there but it just is not shimmering if you know what i mean. as far as i know the output tubes where checked but not replaced. i think i needed one 12ax7 when they went over it. also if the are output tubes that are needed any suggestions on what is the best to go with brand wise. thanks.
What you are describing is a perfect example of old tubes. The lack of interest in listening because the stereo is too mellow and uninvolving. New tubes would make the sound more vibrant and compelling.

Of course something else could be wrong but I doubt it

all the best...

The dulling is usually indicative of the need for new driver tubes as you seemed to suspect. You might also check out the bias of the output tubes - they could be getting old as well. Consider EI's as replacements for the 12ax7's - they are chap and excellent, but allow for potential quality control problems and buy a couple extra. Have no recommendations for the 7591.
when they start hanging out on the corner, getting tattoos, beating up hookers, and slinging dope for a living. that is a good indication they are going bad
When you don't shoot as often as you used to?
don't you have access to a tube tester? that would tell you, although I agree that what you're describing fits worn out 7591s. sells 7591s ($20) but I have no idea what the quality/reliability of new production 7591s is. the high prices people pay for older 7591s suggests that the new ones aren't as good as the old...