I 'll believe it when I see it
you won't believe anything until it slaps you in the face and calls you a liar
I'll believe it when I see it, and even then I'll probably think it's a prank.
You said you'd pass the test? I'll believe it when I see it, and I'll probably fail it.
He said he'd quit his job? I'll believe it when I see it, and I'll probably laugh in his face.