One that’s relevant to what you’re building and the Elixir tools you are using (Phoenix? Nerves?) 
Seriously tho, maybe a different way to look at it might be “How would you find or recruit an experienced Elixir developer?”
That may vary for person to person but a good place to start might be:
- See if they’ve released any OS projects and their general OS activity (and review the quality of it)
- See if they’re a blogger or conference speaker (or even podcaster) (do they communicate well? About topics you feel are in line with that you’re looking for?)
- See if they’re active on places like the forum (again, do they communicate well? Are they amiable? Have they got a lot of accepted solutions? Are they the type of person you would want to work with?)
If you want to hire someone special then you probably should put just as much work in trying to find them (if not more) than you might expect of them to impress you at an interview…
