I 'll believe it when I see it - you don't believe anything until it's actually real and not

you don't believe anything until it's actually real and not just a dream

You said you'd get an A on the test? I'll believe it when I see it, and I'll probably still be confused.

He said he'd stop stealing my snacks? I'll believe it when I see it, and I'll probably still be hungry.

She said she'd stop talking during class? I'll believe it when I see it, and I'll probably still be distracted.