One idea would be to come up with a secret value, then put it in an envelope for a while. Later on, you'd open it and publish the result. If anyone shows up and knows the secret before you open the envelope: maybe they're a time traveler! But while this might be convincing to you, it wouldn't be terribly convincing to anyone else -- they would assume you shared the secret with the "time traveler."
And this is effectively what Hawking did: the secret is just the location of the party.
(Obviously this isn't foolproof. If a reliable government agency prepares the envelope and makes sure nobody opens it, they could convince themselves you're legitimate. Which is why you need to do this as a fun, unserious joke.)
ETA: Also, if you wanted to make sure the government didn't torture the plausibly-deniable "time traveler" anyway, you'd swear everyone to silence and spread the rumor that "nobody showed up." How would we ever know? ;)
Better : people have to Guess on day 1 a random value generated on day 2, and the winner would be awarded with a Big amont of money. Privacy granted. Lottery. Lottery IS a Time traveller detection device.
And this is effectively what Hawking did: the secret is just the location of the party.
(Obviously this isn't foolproof. If a reliable government agency prepares the envelope and makes sure nobody opens it, they could convince themselves you're legitimate. Which is why you need to do this as a fun, unserious joke.)
ETA: Also, if you wanted to make sure the government didn't torture the plausibly-deniable "time traveler" anyway, you'd swear everyone to silence and spread the rumor that "nobody showed up." How would we ever know? ;)