A related expression is "to call someone's bluff." In poker, this means to challenge the person show their cards and risk losing. In other situations, it means to challenge the person to reveal the real situation. "He was acting like he could drive a race car, but when she called his bluff he admitted that he had never actually driven one."
P.S. This post, and all others related to poker, are dedicated to Vassilis, AKA "Nick the Greek," a wonderful writer and friend, who always has an ace up his sleeve.