How do you know when to change the tubes of an amp?

I my limited experience, barring a catastrophe, like the tube actually blowing up or something, you will hear kind of a static coming from either one channel or the other. Swapping tubes from the left side to the right and having the noise follow that swap clearly indicates a tube is going bad. If you have only one tube to worry about, obviously a swap of channels would not work, I would recommend having a new spare on hand to be able to swap in for it to diagnose the noise. If any noise goes away, you know the other tube was bad. Getting a tube tester is always a good idea to be sure of the strength of the tube scientificly, but using your ears will work also. That is if you don't mind your amp being your tube tester.

Good advice.

I've been using the same tubes in my amp for over two years now with no issue. I do have a couple gain tubes that have static in them but haven't tried burning them for a while to see if they clear up. Thank goodness they were a lot purchase. If I'd have paid $50 for one and it was bad, I'd be po'ed.

You can get a direct short in a tube which will smoke your amp. That is rare though.

I change them when they go out.  Sometimes they'll act up a bit before they die, so I'll change them, but most of the time, they just stop working.

I am surprised no makes an inexpensive tube tester anymore. I have been reluctant to buy a used one they are all big and clunky. Usually you can hear the tube when it starts to go south lose much of its luster. But its like Erick said they usually just stop working. IMO it still would be nice to have a tester to check them every once in awhile.

When the bass response goes South, and you have to turn the volume knob up and up.....it's time to start looking for a replacement.

Steve