I would add, work together with them on a problem that is completely out of their comfort zone and/or background. See what kind of questions they ask, how fast

I would add, work together with them on a problem that is completely out of their comfort zone and/or background. See what kind of questions they ask, how fast they recognize a dead-end, and whether they learn from what you are trying to explain. If you have a hard time teaching them something new in an interview, probably it's not a good fit.