calling someone's bluff

To bluff means to deceive or to lie. When you tell someone he's bluffing, you're saying you don't believe what he's saying. Similarly, when you call someone's bluff, you're exposing the person's lie or deception, and you're basically telling the person that you know he's lying. This could also involve making the person prove that what he's saying is true. 

In each of the following examples, someone is lying and another person is calling his or her bluff.
1. He insisted that he was at the party, but Sarah called his bluff. She was sure he wasn't there because she had looked for him all night.
2. He tried to reassure everyone that he had finished the project and that he had just left it at home, but his manager called his bluff when she told him to go home and get it.
3. I told them I'd seen the movie already because I didn't want to watch it with them, but someone asked me something obvious about the movie and called my bluff.
4. They shouldn't have said they could play well when they actually couldn't. We wouldn't have had to call their bluff and kick their ass.
5. The reporter called the politician's bluff by asking which charitable organizations he had specifically worked for. The politician took awhile to answer because he actually hadn't worked for any charities.

vocabulary:
kick their ass in sentence number 4 is an informal and somewhat vulgar way of saying they beat them or defeated them.

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