There's a little tiny symbol in the lower corner. You can see it on the Troll Slayer and Hired Swords cards.
It's a tiny wreath, I think. For the core set. Aww.
For those of you who don't play card games, it's common for each card in a set or expansion to be marked with a unique symbol. You can easily exclude cards from sets that you don't want to use. I don't see why you'd want to do that with an RPG, though, so I'm not expecting the wreath symbol to have any function.