I have only a fuzzy idea of how I would hire for a DevOps engineer, in a way that doesn't boil down to a conversation and a keyword search over specific technologies I already know and hope they know too. I trust that judgment, but it won't scale to hundreds or thousands of CVs.
Blog posts, technical articles, etc. from companies who have successfully done this would be most welcome.
I'd suggest contracting a consultant, or maybe a devops person you know who can moonlight, who can understand your needs and help you find the right person.