Should we question a worker we suspect is gaming our reward system?

Q. We encourage our retail staff to go above and beyond their duties to please and impress our customers. To that end, we reward them gift cards to various restaurants each time a customer comes to our service desk to compliment one of our employees. One of our workers has landed several of these rewards, but I have a suspicion that the “customers” are his friends, and he’s gaming our motivational system. Should I come right out and ask him?