RE: Twitter
For some celebs, you can know for certain that it's really them. It says it is a verified account. Just look at Britney Spears, Demi Moore, Ashton Kutcher or Justin Bieber's twitters for example.
If celebs are giving their twitters on their official website, facebook page and/or myspace (like it is Axle Whitehead's case), then it's probably real as well even if it doesn't state it's a verified account. But as H&A actors aren't as popular worldwide as Britney Spears, Demi Moore etc, I don't know if Twitter verifies their accounts .
I don't really know how it works: whether it's Twitter who contacts the celebs to ask if they have an account to avoid impersonators or whether it's the celebs who contact Twitter to let them know their account is real. I also don't know whether they only mention it's a verified account for US celebs and not foreign ones. (They did it for the Veronicas but they're famous in the USA too). But if they're not doing it for non US celebs, I think it's a big mistake. There are impersonators everywhere and some people are (a bit) naive and get fooled easily.
^^ I'm sorry that I can't answer your question but I thought I'd share this in case someone was wondering about US celebs' twitters.