I call bullshit
A loud and proud way of saying you know the person is full of crap and you're not going to let them get away with it.
"You said you did your homework. I call bullshit, your paper says 'I love pizza' in crayon."
"He said he got a 100% on the test. I call bullshit, I saw him eating a taco during the exam."
"She said she never failed a test. I call bullshit, she got a D and a note from the teacher."