It's important to determine whether or not the person is qualified and capable. While questions have their merit I'd recommend a more creative exercise.
If you have the means, take a trip half-way across the world and live in close quarters for 3 weeks together (while you work to get the business off the ground). If you can't stand them by the end of the trip; walk away.
I've been working with my cofounder for 15 years now and together we've accrued more than ~$30M in combined revenue from a handful of projects. Being able to argue, disagree, make a decision, move on and then grab a beer after work like nothing happened is paramount. Your personal relationship with this person will be the first domino in every decision you make.
Clint Eastwood was on some talk show long ago, and he was asked about relationships, girlfriends, etc. He said he tests potential girlfriends by going on a three-hour driving trip. If they still have anything to talk about at the end of three hours, they have potential. I think that would apply very well to cofounders, too.
And a simple test of how well two founders work together is to take a two-person canoe down a course that demands teamwork. Be sure to switch off who is front and back. I've seen couples descend into cat fights in a canoe.
This 1000%. I've been with my co-founder for about 10 years, 3 companies and one acquisition. We're actually related but that didn't matter cause we didn't know each other THAT well. Being able to argue and have a beer after is the key here because it shows its not personal and that the other person or you care enough about your idea to defend it.
If you have the means, take a trip half-way across the world and live in close quarters for 3 weeks together (while you work to get the business off the ground). If you can't stand them by the end of the trip; walk away.
I've been working with my cofounder for 15 years now and together we've accrued more than ~$30M in combined revenue from a handful of projects. Being able to argue, disagree, make a decision, move on and then grab a beer after work like nothing happened is paramount. Your personal relationship with this person will be the first domino in every decision you make.