I 'll believe it when I see it
you wait for proof before you even think about believing anything
He said he'd bring pizza? I'll believe it when I see it, and I'll probably still be hungry.
She said she'd stop cussing? I'll believe it when I see it, and I'll probably still be annoyed.
You said you'd do your homework? I'll believe it when I see it, and I'll probably still fail.