I am just a moderate tube user these days, but first thing I check for is any audible noise or hiss. Also if anything sounds unusually different in any way, tubes are one of the first suspects. I keep a few spares around so that I can a/b regression test individual tubes at a minimum and replace if not sure.
Noise is easy to identify but other sonic anaomolies not so much. A good quality tube tester like Atmasphere recommended is probably the only way to be sure. Not as common anymore these days, but depending on where you live, in lieu of owning and using a tube tester yourself, there might be shops nearby that keep a tube tester around and might be able to test them for you.
Noise is easy to identify but other sonic anaomolies not so much. A good quality tube tester like Atmasphere recommended is probably the only way to be sure. Not as common anymore these days, but depending on where you live, in lieu of owning and using a tube tester yourself, there might be shops nearby that keep a tube tester around and might be able to test them for you.