First, as previously mentioned, please do not make your decision based on the individual's social media followers. Instead, you might want to look at some of the growth they've helped to engineer in their previous jobs. For example if they tell you that they helped a company grow from 500 to 50,000 twitter followers, that is pretty impressive. However, even then it is important to take those numbers with a grain of salt. Ask how many were purchased vs earned. Ask what specific techniques they used to gain those followers and how they would apply those techniques to your company. Try to determine how much impact that specific person had on the growth.
Second, just like many people suggest assigning a coding project to a candidate to determine if they fit with the company's engineering style, you can assign a project to a marketing candidate to see if the work they do fits with what you're looking for. Ask them to write a sample blog post for you. Ask them to write a sample email they would send to potential customers. Ask them to outline some ideas they have for how to market your company and how they would implement them.
Third, if you want to attract good marketers, please don't treat them like second class citizens. There is a large resentment among many in the tech community towards "Business" people and "Marketers". If you and your company show similar resentments, the person you are interviewing will be turned off.
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.
Alternatively, find someone who can write good copy. You don't necessarily need the person to be tech savvy as that's already covered.
From my experience you can get a lot more mileage from a copywriter. Your website, your UI, your blog, your emails, your lost password reminder, even social media - all of it can benefit greatly from good copy.