Start with people who are inquisitive, and interesting. Look for designers who can communicate quickly in pictures, someone who can draw well. It’s a skill that can be applied across almost every interaction a designer has with others, and save time and make the product better with every deliverable.
Screen for empathy, and an interest in other people and their needs. Empathy is the base raw emotion that connects action and experience in the process of designing software. The objective is to serve others.
A knowledge of front end code is good, it’s the medium we work in. That alone does not indicate an ability to design a process. Look for people who can prototype in code as opposed to relying on animation tools like Sketch or InVision. Demos have their place, but a full understanding of the product and an ability to mock it up quickly goes a long way to selling a concept across the organization, and demonstrates that the design team knows what we’re talking about.
Software Design is a relatively young profession. As software design has evolved many of the best started their careers as engineers, architects, or even negotiators. One of the best designers I’ve seen earned their bachelor’s degree in philosophy.
The results of the team are more important than those of an individual. So look for candidates with project experience that demonstrates an ability to collaborate with others successfully and on an ongoing basis to make products better.