that’s simple!
just plug amp into ac power (or current) measuring unit, and dial the volume control.. class A power consumption is not sensitive to volume, while AB will consume significantly more with volume increase.
Class of operation for Tube Power Amps
My understanding is tube amplifiers can be ran in Class A or Class A/B operation just like Solid State amps. MANY tube amps do not say what class they are running. If they don't say this in the specifications do you just assume the are Class A/B. How can you tell?