Though this article is focused on hiring developers, it applies to any technical hire, anyone who needs to be able to grasp a complex topic and become useful in that area relatively quickly.

One of my favorite pieces of advice:

Much more important than what they know is how they learn it, and how quickly. You are looking for somebody with a track record of learning new skills and applying them successfully. Talk about their career path, and look for evidence of increasing responsibility (which is related to, but not the same as, seniority). Remember that anybody you hire will expect raises every year: somebody who isn’t getting better all the time is going to become [a] worse and worse value unless their skills increase in value, too.

Terrific foundational stuff.