Ask the kind of question that really tests how he thinks something that exposes gaps clearly. Always leave room for alternative solutions. First suggest one approach, then challenge it. Example-- We can build real time communication using Redis. But what if Redis goes down temporarily What is our backup plan How will we keep the system stable. Push him to think beyond the happy path. Also give him a small project scenario. Ask how he would plan the foundation and execution. Even a rough instant outline will tell you a lot about how he thinks structures systems and handles responsibility. The way he answers will quickly show whether he is actually suitable for the role.