Intellectual and creative vigor.
For the former, I usually ask various questions about food - likes and dislikes - and about their experiences with food, specifically, dining experiences they've either had or created that are especially memorable, and why so. I'm mostly interested in how they tell these stories rather than their content - the vibe, if you will.
For the latter, I'l hand them a steak or a piece of fish, or an eggplant or a few portobellos and tell them to cook me whatever I've given them and include a sauce. Mise en place will be conventional but with a few extra things and some oddities there. Other available items they can use (or not) I'll assemble nearby on the line. Then I go do something somewhere else in the kitchen where I can glance over periodically.
Rote cooks give themselves away, in most cases, pretty quickly. They will have numerous questions (virtually none of which I'll answer); creative cooks rarely will ask a thing. I look at how they assess and approach the mis en place and the assembled potential ingredients (rote cooks will usually ignore the unfamiliar or unknown) and I watch their faces. It's often easy to see rote cooks thinking backward to something they've done many times before as their intent seems to be to choose something familiar and comfortable, so as not to screw it up. Creative cooks look, well, creative. Most of them will glance here and there and then just start prepping or cooking. Their expressions usually tell me they're winging it - making decisions as they go - but they look utterly comfortable doing so.