No one knows who is Satoshi Nakamoto so for me that is impossible to prove if someone pretend to be the true satoshi nakamoto.  But if satoshi sign message in her bitcoin wallet I think many people will believe in this because that is the only way to prove that satoshi is alive.  Because wallet are hard to crack thats why I will believe if satoshi create signature on their wallet.  And even here in Bitcointalk I will believe also if satoshi open her account again and post that he are alive.