If you're technical, I think anything where you actually get to work on projects with people is going to have a lot more potential than just talking. So hackath

If you're technical, I think anything where you actually get to work on projects with people is going to have a lot more potential than just talking. So hackathons, open source, and other things in that vein. The difficulty here is given you find another technical co-founder, at least one of you needs to be willing to jump hard into sales, marketing, and administrative stuff. But assuming you have that, two strong devs who enjoy working together can make for a formidable team, and it doesn't really take that big of a project to know whether you like working with someone.