The only way to tell if you work well together with someone is to try working together. At co-founder matching, we recommend that before jumping into a full-blown co-founder relationship, you work on a trial project together first. The general YC advice is that by default, you should just split equity equally. But again, do you risk it until the point you can? And then you just go.
How do you find a co-founder and build a great partnership? YC's Catheryn Li (who also built our Co-Founder Matching tool) and Visiting Partner Divya Bhat cover the importance of having co-founders, how to get & vet one, and the ways to build a successful working relationship with them.
Apply to Y Combinator: https://yc.link/SUS-apply
Work at a Startup: https://yc.link/SUS-jobs
Looking for a co-founder? Sign up for Co-Founder Matching: https://www.ycombinator.com/cofounder-matching