There have been many times that I've needed more help on the technical side, moderation side and a combination of both. Treat this like a job interview and you are the hiring manager. There isn't a generic list of questions that we can provide that will be applicable to you all the time for all positions (or to all other sites). Determine what this person will be doing for your community and build your question list around that.
For the technical side, life is a bit easier because I can present a problem that we've had to solve in the past and ask for their solution. With this question, I'm not looking for an answer that mirrors the result we came up with. I'm looking for an answer that can think through the problem, suggest a rough solution, identify potential side effects with that solution and then adjust the suggestion if needed. I'm much more interested that they can think through the technical problem we are trying to solve and apply more than a simple band aid that will push the real problem 6 months down the road. If they can provide previous work examples (GitHub, their own web site, etc), that is always helpful too. Most of the time, I wasn't looking for something related to what I needed help with. I was looking to see if their code was understandable because they'd be doing this one thing for me and my existing team and then be done. We'd have to maintain it.
Dealing with the community aspect is more challenging. I've gotten help from both community members and non-community members. Both provide benefits and pitfalls.
A community member is more likely to understand pain points we are already experiencing. They may have potential solutions too. The down side of a community member is possible baggage they bring along. Is this a user that many know and respect? Will that respect remain if they need to handle moderation actions against their friends? Is this a user many don't respect? Do you respect this user?
A non-community member can provide an outside perspective. They don't know "we've always done it that way", because they are new. The community may see this as a positive or a negative. Will this person be able to handle both types of feedback?
If you are looking for a technical user that will be doing a lot of interacting with the community, then a combination of the two are needed. Can the user suggest a technical solution to a problem you've already had and then defend that decision when it negatively affects some users? Are they able to take a problem users are having, engage with the users to get more details, then build a technical solution? Or, even better, determine if a technical solution is needed instead of either a policy change or just user education?
A generic list is hard to provide, but hopefully the above provide you with some areas to think about so that you can focus your questions. I've found that "real world" questions are much more helpful than abstract questions. Provide the user with a problem you've already had and see how they would deal with it. This lets you see their thought process and opens up them up to discussion about why. Even if you disagree with their position, if you can talk about why they picked on route over another, you can gain insight into how they will work for the community.