Fraud detection is winning an entire match, without letting your opponent win a single round. If your opponent lands even 1 hit on you, you will not get this achievement.
At the next-to-last round you will see a Fraud Warning appear. When you win the final match the "Fraud Detected" stamp will appear, and this achievement will be awarded.
If boosting with a friend, the friend can choose the YOLO gem, which will automatically put them into a Fraud Warning (1 round til victory) state.