I think merely asking this question would be qualification enough.

As I suggested in another comment, it might be in general bad advice to try to get people to ask clairfying questions about non-contextual interview problems. If you want to measure social adeptness / willingness to ask for clarification -- which are important! -- consider interview questions that have a "role playing" component or actively invite the interviewee to ask questions.

In my experience, clarifying questions are important. In this case, for instance, we have two different problems (well, not too different, but still).