Same way you find out if someone really is an account manager, proposal writer or any other position you are hiring for -- do your homework.
FIRST before you even start looking for a social media expert, find out what other companies put in their job postings looking for such a person. Online job search sites such as workopolis.com and monster.com are the big ones, and if you do a search, chances are you'll find postings for social media specialists.
SECOND look-up online anything you saw in the job postings you didn't fully understand. Acronyms, software tools, etc.
THIRD I'd Google or Wikipedia the top social networks, and explore them. Learn a bit about what they are like, and bookmark them for reviewing candidates profiles later.
FOURTH take all the information you've collected, and make it your own. Think about how your company can best use social networking. Make a list of things you want and must have.
FIFTH take your list and create a job posting. Filter responses against your list of needs and wants, make a short list of prospective candidates and bring 'em in for an interview.
SIXTH before hiring someone in a social networking expert/specialist role, check 'em out. If they can't promote themselves well online, just think what they won't do for your business.
SEVENTH check out their references -- if they were doing a similar role at other companies, were they any good?