How to tell if the tube needs replacement?

I don't have any experience with tubes but my CAL Alpha has 2 12ax7's in it's output stage.What to look for in the sound when the tube is going?As you know some days even the best systems sound bad.Thank you.
Inconsistancy in sonic quality is the first sign.

Sorry, had to say it.

Different amps do different things when the tubes age.

The general signs are mushy bass, loss of high freq. detail,
smeared midrange imaging.

Maybe someone who has experience with an Alpha can chime in.

My experience with Cal Alpha is the the sound will become dullish - you will expereince some HF roll off and lose some of the sense of spaciousness/air. Keep a spare pair of tubes around and plug them in when you feel this is occuring - if nothing changes then it was your mood, but if it brightens up (in the best sense) you needed new tubes.
If you hear Strain or distortion . I do try to retube stuff every few years especially power tubes.