I'm asking this because of this answer to How did Gordon Freeman survive the entire storyline without a helmet?. The author claims to be one of the creators of the game. He has no other answers or questions, or other SE accounts linked. I edited the ID out of the question until he can prove it's him. What should (or is) scifi.SE 's policy about this?
This is something that basically anyone can come along and claim.
I'd personally want to see some kind of independently verifiable reference; a blog that is known to be the individual in question referencing the same information or even linking to this question would be a good example. In the absence of such, a reasonable degree of skepticism (this is the internet, after all) seems in order.
No proof is necessary to prove someone's identity. Why? Because what proof could they offer?
Really, the only "proof" would be them posting a photo of them holding up a handwritten sign saying "I am user123294 on scifi.se!", and that only works if there are publicised photos of the person in question (which is hardly a safe assumption, particularly if they are part of a larger team that collaborated on a project like a video game).
Even that could easily be photo-shopped.
Finding an official blog or other publication that agrees with the answer isn't "proof", either. They could have just read the blog, and then posted their own answer while pretending to be the author of the blog.
A better question, though, is why would we need proof?
It is up for each user to judge an answer and determine if it is helpful. If you don't believe that William Shakespeare really logged into the site just to answer a question about the inspiration for Puck, don't upvote the answer.
If you believe that the explanation seems plausible, and that the person claiming to be Danny Rubin seems legit, upvote it if you want.
What we shouldn't be doing is demanding that everyone who claims to be involved in the creation of the work a question is about "prove" their identity, especially if those demands include thinly-veiled accusations that the person is a liar.
Let's say the guy who answered the Half-Life question really is exactly who he said he is (and I find the argument that typos and grammar mistakes mean that it is unlikely that a person really is a professional illustrator and game designer more than a little questionable... Especially given how easy it is to make typos, and how difficult it is to correct them, when using a cell phone). Here he found a community ostensibly devoted to discussing scifi and related topics. He's offering what to him is a good-faith contribution, and what's the response? Multiple comments criticising his writing, accusing him of lying, and demanding that he put in more time and effort to provide proof that would only be subject to further skepticism.
Assuming he really is who he says he is, why on earth would anyone expect him to come back?
If you see an answer from someone claiming to be in a position to provide "word of God" answers, yet you have reason to suspect they aren't telling the truth, the most you should do is put in some research to see if you can prove they aren't who they say they are. That should generally be easier than them proving their identity (which isn't to say it will actually be easy).
If you find some fairly strong evidence that they aren't who they say they are, you can post a comment explaining what you found, and why you believe it is cause for concern... politely.
And that's really the main point. Be polite.
Even if the person is lying through their teeth, and you know William Shakespeare personally, and this person posting is clearly no William Shakespeare, that is not justification for being rude to them.