What specific tactics,questions did you use to measure how/if people can get shit done,have a positive mental attitude, and actually have a passion for your concept. There are many other things that go into identifying whether a co-founder is a good fit or not, but I assume it should be somebody like me, which is very difficult to find.
Understand their background and ask them to give examples of what they did. Ask questions that define their character. Ask them what their goals are and why they would want to be involved in starting a company. Do these things align? If they do, agree to do a three month trial period to see if you can actually work together.