How to tell from which expansion a card is?
I've not handled Descent cards myself, but in my experience most games with card-based expansions &mdash; Fantasy Flight games in particular &mdash; tend to have small icons on their cards which indicate which expansion they come from; I usually find this in the lower right corner of the card itself (no icon typically means it comes from the base set).

From my quick skim of the instructions, that does appear to be the case here (see, for example, page 3 of the Lair of the Wyrm expansion rulebook, as well as page 3 of the Trollfens rulebook).