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.

Indication of possible CCG prospects for ability cards?

The symbol may also be used to differentiate between versions of the same card when an updated card is released with a new expansion to the game.

Yep. They do this with the Warrior Knights boardgame. All the Crown & Glory expansion cards have a little Fleur de Lys (at least that what it looks like) on them so you can easily add or remove those cards...