If Satoshi returns back with his forum account here and tells who he is, then its very likely that most people will believe that he is the real Satoshi.
Note that his account is banned from this place, according to theymos. So, even if he tries to login, he won't.
Moreover he can also give a signed message of the Bitcoin address that he used here. This will give more believe from the people that he is the real Satoshi.
Satoshi never disclosed his address(es), so any signed message from one of his "probably his" addresses isn't a proof of identity, but rather a proof of ownership of the private key. The only action that can act as proof of identity is to provide a signature from his never used PGP key[1][2].
[1]
https://bitcointalk.org/Satoshi_Nakamoto.asc[2]
https://web.archive.org/web/20090131115053/http://bitcoin.org/Satoshi_Nakamoto.asc