How do you know when tubes are done?

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.

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