I think you need to look for people who are dedicated to the project, willing to work hard, be flexible, and learn as they go.
Evaluating pre-product market fit is work a startup needs to do so at the point you speak of it's still a gut level estimation you need make when deciding to join.
I would tend to shy away from bluster and ego. I would need to see people more excited about making the product great than the big bucks they expect to get when they cash out.
Ask what needs to be figured out and where their weak links and hurdles are.
Ask them to tell you what they don't know. That is something all startup team members need to do their best to understand because here there be monsters.
> Ask them to tell you what they don't know
And what if the situation is of them not knowing what they don't know? Unknown unknowns are there....
Most likely that is the case but the point of asking is to see how well they've thought out the product and to uncover areas that may need propping up.