How do you know when tubes are done?

to Millercarbon,David doesn't have regular products. Each one is different to some degree. He's always trying to get the best sound. The best sound I heard was from a SET design but bass was weak. He's really more interested in circuit tweaking and very minimalist designs. On/off toggle and volume knob only. He has to sell them to recoup the money spent on his 'hobby'.He might still be on Facebook but I don't use it. Can someone check, please?
I just got a pair of Klipsch RP-600M horn speakers. Very sensitive.Good sound and a good bargain, I would say

When the timer goes off your tubes are done.

I’ve worked on tube gear for years, and judging by physical signs is not reliable. Especially with power tubes. Running a power or integrated amps on weak output or driver tubes risks damage to the amp. Amps with bias adjustments should be checked periodically. A significant change in voltage can be an indication of a tube in decline. For small signal tubes, the only truly reliable way is to use a tube tester. Ideally a Hickok or B&K. These can be expensive, but you can still find old Sencore "Mighty Mite" testers on ebay now and then for around $100 or so.

1947...courtesy of Bell Labs.  1967 courtesy McIntosh Labs when Frank determined his solid state amplifiers sounded better.  ;> )

Thanks Pixelreffic,
I will trundle off to my local repair shop.
And get the rest of the amp tested as well.
I am not a tube geek. I know nothing.