Ask friends who have technical skills in this area to help you out. Don't just trust the impression the people make. Evaluating a persons technical expertise isn't easy for anyone not deeply acquainted with the given technical area.
If you create a listing of the Results you want to get from this person as well as the critical things they themselves must do or "own", you can have them do examples of these aspects and see the quality first hand.
There are two strategies that people are increasingly citing. The first is to do Paired Creation, where you have two people work on the some Result, together at the same time. It can accelerate turnaround and produce a better net effect.
The second is to set up a project that allows you both to experience each other for 60 - 90 days. this keeps you both motivated to collaborate.
This kind of work is both Art and Science, so there are a lot of ways to interpret results.
Many of the posts here reference talking to others about the person you intend to engage. It is important to research your key people hires, so that you can make the most informed decision possible. Hiring them is no guarantee. It is very much like dating. You check for chemistry, shared values, work ethic, interests, abilities style and direction. In the end it is a relationship with you and them as well as a relationship with them and the company you are working to build.