I guess it's too late for any input, but just in case:

• Ask them to compare django to other frameworks, likes, dislikes
• Ask them about a few packages they used before
• Have they written their own migrations or package
• How would they deal with a slowly loading page?
• Do they know about SQL? On what level? Have they used advanced features like views/materialized views, triggers, lateral joins, Subqueries. Can they tell you about different kinds of joins.
• Figure out if they know the basics of HTTP and forms
• Ask them about html and css stuff
• How would they go about implementing a telephone book application? (or any other pretty simple application). Have them sketch a rough plan on what to implement and test when. eg. Make models first, then a view and a template, etc.
• Have them explain what CBVs can help you with.

I know, it's vague. You want to get a good picture about how deep their understanding of the web in general is and also see if they have used django beyond very simple tutorial projects.