Class of operation for Tube Power Amps

How to Tell the Difference: 

1. Look at the number of output tubes: If the amp has only one or two output tubes, it's more likely to be Class A. If it has four or more, it's likely Class AB. 
2. Check the power consumption: If the amplifier draws a lot of power relative to its output wattage, it's more likely to be Class A. 
3. Observe the heat: Class A amplifiers run hotter than Class AB amplifiers. 
4. Consider the design and circuit details:Class A amps often have simpler designs and fewer components. 
5. Consult the manufacturer's specifications:The manufacturer's documentation or website will usually specify the amplifier's class of operation. 

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. 

@westcoastaudiophile It's not simple if you are looking to buy an amp and don't have access to it.

@gkelly Thanks for the reply. I was looking at 3 different tube amps and none of them address the class of operation in the description or specifications. I do know almost all solid state amplifiers do address this.

willywonka what are the 3 tube amps you're looking at?

gkelly nailed it.  

If a mono tube amp has just one tube or a stereo tube amp has just 2 tubes it isn't likely to be class a, it is absolutely class A. There is no other choice due to single ended circuit topology.

Other wise it's not absolute but the odds are any push/pull amp(two or more tubes per channel) is class AB. In fact any push pull amp solid state or tube is unlikely to be class A at very high powers although it may be what's called rich class AB meaning it stays class A to a power high enough it almost never goes out of class A into class B.