A pair of liar's cards

Exploration Stage: Red Rose Cafe
From: Detective
Description: No one thinks Master F won an honest game. Card cheaters are the best winners in town. And Martha senses the deception. But if this is true, then Master F's victory is null and void! We must gather evidence that Master F. committed fraud. Then the card cheater's fortune would return. Was Master F using marked cards?
Completed: That's it! A real card with a real mark on it. Looks like an example of card fraud. When I was just a cop, these cards hurt me a lot.