Maybe. If I were the real hacker wanting to prove it, I'd maybe leak a few celebrities without previously leaked SSNs. I think at least one of them would unintentionally verify it for the public by getting upset and ranting on Twitter. If that didn't work, I'd try something else.

Disclaimer: I'm no expert on security. I just like applying my things-I-might-do technique.

Well, I suppose you could always try applying for a credit card with the leaked info and seeing if you get approved!