Tried a project with one and saw we worked great together. Anyone you are not sure of, try working with them on something and see what happens. If you are still not sure you haven't lost out. Plus, working on stuff is quite fun.

You really need to test all aspects of the relationship with cofounders. So deliberately acting angry to see what they do or needing help and seeing how they respond are examples.

Spark is one thing you notice if things work out. Good luck!