It depends a little bit on your company stage. Marketing manager at a startup may own a bunch of channels and be pretty boots on the ground for getting things across the finish line. They are probably 80% IC, 20% mgmt (e.g. one direct report, or external consultants). At larger companies a MM may own one channel and play more of an oversight role regarding channel performance, channel management, and direct report management (but still, 1-3 and not more IMO).
Baseline, your MM should be the in house expert for the channel(s) they own and should be able to both manage and execute within the scope of the role. Whereas a director may oversee all of digital and have the MM roll up to her. A VP may oversee all marketing and intersect with strategy and sales.
Key skills I’m looking for:
If this role is a step up for them, then look at projects - even if small - that they owned in previous role. Do they know how to take on a program and really own it.
They need to be learners and gritty, regardless of the size/stage of business.
They need some left-brain skills. Should be able to figure out the levers that make things work and optimize for those
Cross-functional. Can they work with others and manage people - even at a small scale - to drive things forward.
Strong writing skills.
Not all channels are equal. A content marketing manager is going to be totally different than a PPC/demand gen manager than a creative role. So, I would focus on hiring someone with the general skills above plus the detailed skills for the respective channel(s) they are going to own.