The act of calling bullshit: When one person says something that another person is not in agreement with, that second person may "call bullshit" on whatever the first person said. By doing this, they are expressing their disagreement with what the person said in a humorous and yet serious way.
Person1: "Of course I don't drink! I'm not 21 yet so that's illegal."
Me:" Umm sorry, but I call bullshit on that. I was with you last night at that frat party and you were far from the perfect little sober college student you're pretending to be right now."