I'll bet
When someone says something you know is a lie, you say I'll bet like it's the last thing you'll ever say.
He says he never eats cereal for dinner. I say I'll bet. He's eating it right now.
She says she doesn't need sleep. I say I'll bet. She fell asleep in class.
He says he's never been in trouble. I say I'll bet. He's got a detention slip.