It's more important to consider the process the designer brings to your organization than any one piece of work and that process can't be explored with a task in an interview. Design is solving problems that connect users to products, that can't really be performed in a vacuum.
If you don't like a person's work, then move on. If you like what you see, consider these questions among others:
1. How and why did you decide to create this way?
Did the designer perform some research, did they use/create personas, were they working from an existing style guide (just how much of the work is theirs).
2. How long did it take you to create this?
Are you fast, slow, how long will it take to get what I need done, done?
3. Did you think it would take more or less time?
How accurate will your estimates be? Are you an alarmist or calm and collected, you know what you're doing?
3. How many iterations did it take?
Do you listen and understand what your customer/clients actually need (versus say they want)?
4. Did this require any research on your part? What kinds and where?
How much do you really know versus how much do you have to google it? Are you a copy cat? Did you need to employ standards? What did you learn and what can you teach us?
5. How would you change it, knowing what you know now?
Do you really think you're all that? Can you edit your own work, or are we going to have to be heavy handed and direct you to do everything we want?
6. Do you have any data/analytics on its use?
How much do you care about how it was received and used? Just how invested in your work are you? Do you understand the value of analytics, usability, etc?
The point: focus on what was done in the past, what's changed (what have they learned), and what they'll do for you with your actual products going forward. So again, consider that the process they bring to the table may be more important than any one piece of work they've produced.