I'll bet
When someone says something you know is a bunch of nonsense, you say I'll bet like you're ready to laugh at them.
You say you can talk to dogs. I say I'll bet. The dog ran away from you.
He says he can read minds. I say I'll bet. He didn't know what I was thinking.
She says she can sing like a pro. I say I'll bet. She sounded like a cat with a sore throat.