The best thing I’ve done in life is chosen co-founders for my coworking space startup that serve a wide variety of major duties. All Active. All important.
The first and the foremost thing you do while screening your co-founders during selection is getting to know them well. Until unless you know how the person behaves overall and in specific situations you cannot move on about thinking his skills. Asking questions straightaway in their face may not be a good idea so get to know them by being with them, checking how to react in tough situations, their emotional reactions, their likes etc. This check will definitely pay you off because this may not matter right now but may be a big factor in future.
Then comes, checking what skillset are you looking for and how much skilled is he/she. Is he skilled enough to earn the chunk of partnership you’re offering. If yes, trust his/her skills and leave decisions to him while working. This generates a sense of trust which is very important in any partner relation.
The last but not the final major thing that I would suggest for you to look is his motivation level/passion towards work. If all the partners are pumped enough towards a common goal, it becomes very easy and smooth to sail through tough situations where alot of hard work is required.
Rest, I think everyone is good enough to set terms of co-founding and partnerships etc. But keep in mind, one thing is very important. All Co-founders need to serve a big purpose in the company and should serve exactly all the major departments like tech, marketing, after sales etc. Questions are welcome.