call someone's bluff
Expose someone's deception, invite a showdown, as in I don't believe they have enough capital; I'm going to call their bluff. This term comes from poker, where bluffing (pretending) that one has better cards than one's opponents is an intrinsic part of the game, and calling someone's bluff means forcing them to show their cards. By the late 1800s it was being applied to other enterprises. Also see show one's hand.