Not always will the number of output tubes dictate if it’s class A or A/B there are a few parallel class A amps with more than one output tube per channel. I know a couple of parallel class A SET’s for example.
I think doing a quick search on the amp in question on the internet will tell you quickly what class type it is. Most amp makers that are using a different class design i.e. parallel, etc. will state that as a selling feature.
Heat won’t really tell you much in a tube amp as tubes need heaters to function and they make more heat than the class type, usually.