15 years UX researcher on product and in agencies here.
Honestly I think for this role the interview should be more a gut check on personality and expectations, after you’ve seen their work and read their case studies.
That said I’d go for a deep dive on the work product from their portfolio that is most analogous to the problems your team is solving.
The best lead/senior designers I’ve worked with share some traits you should look for.
1. They are flexible, not prima donnas— but they DO have standards they’re willing to stand up for.
2. They can fluently explain WHY they made their choices, and recognize and articulate trade-offs.
3. They understand enough about research methodology to be able to internalize research results in their work product.
4. They understand development enough to know what not to do, and when to push the devs to try harder to implement.
5. They’re very quick on the uptake, and can understand the BUSINESS problem at hand, not just the visual or interaction or information problem.
6. They are clear, grammatical, concise writers. Lest you think this is irrelevant, keep in mind it’s often the UXD writing the micro copy.
7. They fully understand design systems, tokens, atomic designs and are HIGHLY organized.
In our world of UIs and complex product interactions, design systems and tokens, it’s much less important that a designer be concerned with aesthetics. The roles of wireframes (typically what I have done in the past as a UXR/generalist) is merging with the design skill. (Personally I think that trend ought to go the other way, but it is what it is.)
