Determine if they have a chip on their shoulder . Do they have something to prove? Where’s the hole they’re looking to fill? Ideally, you’d get someone with obvious talent/drive that is misunderstood by the world. Those types of people will run through walls to reconcile their internal view of themselves with external perception.
If you're the CEO of a fast-growing startup, the mistake you'll make in the next year that you'll regret most will probably be a hiring mistake. Possibly hiring someone established, which you'll feel you should do now you're getting big, and which VCs will strongly encourage.