For one, if the person can continue on the account satoshi started of with here in the forum, I believe that will be one proof even though maybe that could be hacked
That account is locked by Theymos, this kinda raises the interesting question though of whether there would be something to persuade him to unlock it (for the real satoshi - or a good masquerader - if they return).
There will always be doubters also even if the real satoshi does come forward one day and even if the real satoshi does come forward, they shouldn't be given too much weighting in discussions as they might hurt the growth of the currency (there were lots of people complaining about the segwit forks not being "satoshis bitcoin" - even before that Australian claimed to be satoshi).