Interview their achievements and let them explain why, and let them code some features in front of you.
This is the best way to interview and get the best programmers to work for you. Gone are the Microsoft’s days: “Why the manhole covers round?” (brain-teaser). Or Google’s Algorithms questions: “Write me a binary search”.
If he did no real projects with real users, do not hire him.
Resource: Why The New Guy Can’t Code