Non musical noise frequently heard when warming up is one indicator. Drop out, is another, simply non functioning lack of sound or low sound and the worst of all an electrical short or massive failure.
The spectacular shorting or "arcing" failure, which looks like a lightening storm in the tube. The Arcing failure can take out other elements in your amp , I had one such failure, an infant death no less and it melted a tube socket resistor and burned a wire. Another electrical failure is red plating but I don't know if that is related to age.
So if you think they are old you might want to take them out prophylactically. I see a lot of people decide to replace only when the tube won't hold the bias.
The spectacular shorting or "arcing" failure, which looks like a lightening storm in the tube. The Arcing failure can take out other elements in your amp , I had one such failure, an infant death no less and it melted a tube socket resistor and burned a wire. Another electrical failure is red plating but I don't know if that is related to age.
So if you think they are old you might want to take them out prophylactically. I see a lot of people decide to replace only when the tube won't hold the bias.