You ask them about their experience. What projects they've worked on. What things they've created. Then you drill down into the details with them. What was hard

You ask them about their experience. What projects they've worked on. What things they've created. Then you drill down into the details with them. What was hard? What was surprising? How did they solve it? How would they do it differently? Or you do a mock design exercise with them and hear them talk through their process. Are they going to use a relational DB or NoSQL? Do they need a queue or an event stream? What language do they want to use? Why? Or worst case scenario you give them a take home exercise (compensate them!) and then for the follow up you talk with them about their code and the compromises they had to make. Or hell, if they like white-boarding and want to do it, then go for it. But a lot of people (like me) have a tendency to lock up in those situations.