1) Dig into exact contributions to campaigns for past roles. Ask for the logic around decisions. Often marketers talk about their campaigns results but really they were a body in a room with little input. Dig for detail and motivations to decisions and you will find out who is who.
2) Look for initiative. Many marketeers repeat what is always done optimising here and there. Especially for dynamic businesses i'd look for people that see the world independently of their enviroment, have the initiative to shift how things are done. This is someone that will deliver results vs turn wheels for the sake of it.
3) Look for someone that has experience working big and small companies. The big brands will train comms, project management and procedure. Small brands should give the 'getting it done' skills, generalist experience and show they can get technical/dirty hands. Larger brand experience can look impressive but often the work is more project management for agencies and doesnt suit hand on. Likewise if you're business is big enough to run agencies if someone has always been hands on they can be weak project managers and communicate needs ineffectively.
4) Find someone that can analyse data vs report it. There's a big difference in the 2 and the former is surprisingly rare.
5) Avoid anyone that shows narcissistic tendency or is generally 'me' focused. This personality tends to be bad comms people (especially with social) as they see the world from their POV, not the customers.
6) Give marketers product input. They should have their finger on the customer pulse and valuable views on what sell/motivates users. Dont let tech side shut out marketing.
7) Don't go cheap. I often see startups advertising low paid roles a couple years experience, while they pay for experienced developers. Half the price sounds great but they will bring a fraction of the value.
8) Make sure they have a budget. This goes well with the above point. Generally I'd say a 10-20x marketing salary would be a good rule of what you should be looking to spend if bringing a permanent marketing role in.
9) Sales and marketing are not the same thing. Know what you need. They dont often come all-in-one.
10) While not an absolute I usually check linkedin contacts to skill endorsements ratio. Typically this ratio is higher for the people I know are good marketers. I wouldn't make my decision by this but is seems one of those soft indicators.
11) When you interview give a 10min exercise before to discuss and cover up real-time actual skills. E.g. show them some relevant to role campaign materials (e.g. landing pages/EDM's) and ask them how they would improve. Ask them what they would do with $50k. Typically I add in some spelling/grammar mistakes to also look for attention to detail as it tends to be a 'you have it or you dont' skill (like initiative) and not something you can train up.
...hope that's useful...not a rant :)