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.
Hiring a COO isn’t easy at the early-stage and is wrought with more danger than potential success. Make sure you take the time to understand if you are ready for this hire. When it is done right, the impact on you and the company is tremendous.