I don't have William's email address. That's private. But I have read the interview that used to be posted on Aaron Gilmer's site until it was pulled at Palace request. I have talked to people who have dealt with confirmed William fakes. Here are some ideas.
How do you know? At all? You don't, not until you have confirmation. Good guesses are that the person sounds like a Brit (ask a Brit friend or two whether the person sounds like a Brit, or conversely, may be faking Brit). Test to see whether there's something there besides superficial info anybody could have picked up from reading Seventeen. Remember, if you found it there, so could someone else have. Talk to this person for five to six emails. Do they sound like a royal (not your idea of a royal, but a royal) rather than a Xeroxed bio sheet? There is a big difference.
There have been very few confirmed William sightings on the Web. But there have been a couple. I have read the Aaron interview (which we now believe is genuine) and I have talked to others who were fortunate enough to catch him posting on one of the more remote message boards (although this was not confirmed until later and he was believed to be a fake at the time).
Without a degree in language, a background in literary analysis, and a good knowledge of psychology, I can't tell you more. Just listen to the writing. Does it say England to you? That's the best advice I can give. The royals are the center of the land and have been for centuries. That's why they're there. Just listen. And William's been called a fake before. So no telling.
Remember, this is a human being, not a cardboard bio sheet. Shy? Maybe not. Fruit salad? Maybe some reporter caught him chowing down once when he was hungry and that was all there was. Assume everything. Assume nothing. And check with a Brit. Preferably an older one.
And remember, if you've found this page, the William fakes may have too. And the chase goes on....
And how do you know? Maybe he likes Pepsi.