How do you know when 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.

Here's a photo. 
https://www.psaudio.com/article/do-it-yourself/