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.