My opinions, partially based on experience from 55+ years of using tubes, building tube electronics (mostly from kits) and tweaking tube electronics. I'm no expert, but I saw one on TV about 20 ago.
How to tell if a tube needs replacement?
Increased noise in one or both channels
Loss of sharp imaging / soundstage
Tube fails to glow (burned out heater element - "hard down")
Persistent humming in one channel even when a non-connected source is selected
Tube goes microphonic (tapping on chassis can be heard through speakers)
Increasing distortion or output level in a single channel
"Motorboating"
Bright glowing of tubes, particularly accompanied by humming, sometimes LOUD humming (brighter in one tube than others)
Errant tubes can sometimes be identified by switching noisy or microphonic tubes to different channels to see whether the noise / microphonic follows the tube.
