I smell
When you know someone is lying, and you're not going to let them get away with it. It's like saying 'I know you're full of crap, and I'm not in the mood to play games.'
My friend said he didn't cheat on his test. I said 'I smell' and showed him the evidence.
My mom said she didn't eat the last slice of pizza. I said 'I smell' and pointed at her belly.
My brother said he didn't break my phone. I said 'I smell' and showed him the shattered screen.