There is visual confirmation that you have not beaten an NPC (or gotten all cards yet) - kinda the opposite of what you're asking for, but should work aswell?

Any NPC you have not beaten yet will have a triple triad symbol with an exclamation mark above their head and the same symbol will appear on the map.
Any NPC you have beaten, but still have cards to win from will have a triple triad symbol with a star above their head and the same symbol will appear on the map.
Any NPC you have beaten will no longer have any marker above their head or on the map.