That’s a great question. I don’t think there is one perfect answer. However I would suggest that you find someone whom you’ve worked with at some point so you know if you work together well. If you don’t have previous experience working with a potential co-founder then I would suggest you try hiring them for a short project to get a sense. Or even better start something new with them so you’re both equally invested and if it doesn’t work out then you still have your original project.
Thanks - it's been tricky to say "no" and it feels weird to ask them to do a trial before. I think it depends on them as they might feel they are not "good enough" and it requires a certain level of maturity to realize trials are good for both of us.