Keep this in mind: Their understanding of psychology, technology and communication is much more important than how many followers they have on social networks. Someone's social network activity is often misinterpreted as being important in helping grow a company. That is really not the case in most circumstances. Try to find someone who is up to speed on the latest tools, but maintains a healthy level of skepticism to them. They need the ability to help your potential customers become aware of and understand your product. There is a flip side to this as well. A good marketer will be much more than just a mouthpiece for your company. They should be able to communicate the needs of the market back to you as well.
“The aim of marketing is to know and understand the customer so well the product or service fits him and sells itself.”
~ Peter F. Drucker
Great marketers do that. If you're considering one branch of marketing, Promotion and Marketing Communications in this case, you may be missing a deeper opportunity.
Mar Comm is a tool of the trade and is a small part of good marketing, it's the most outwardly visible however. If your quest is for a megaphone you may have better luck seeking a PR or communications type person than a marketer.
If you're looking for someone with a holistic view towards marketing, want them to drive your strategy, and do the heavy lifting then I'd advise finding someone who is very experienced in marketing from a business development type role to assist in identifying and selecting the right person for you.
That may not be the answer you're looking for but I don't think there's a shortcut here that will be successful. We humans are terrible at assessing the skill of others in a field we have no direct experience in.