1) Non-tech co-founders need to really understand their target segment on an intimate level. They may not be able to write all the code, but they should be the masters of the user domain and/or business domain that the product is going to serve. They should know what the right strategic goals of the business should be, given their target user. They should know exactly what problem they're trying to solve, for whom, and (roughly) how.<p>2) They should also have strong, well-informed opinions about UX. Take them to task on what makes a good UX or even a good UI. Press them for examples of good and bad UX design, or have them walk you through a mockup of something you've done in the past. You should be interviewing them just as much as they're interviewing you. They should have theories and opinions on how UX affects <i>business strategy</i>, and vice versa, and not just opinions on what looks pretty or simple. You want to get the impression that this person would have made (or was) a great product manager at a big company, had he or she not been a founder.<p>3) They should be great salesmen (or women), but <i>not</i> bullshit artists. There's often a fine line here, but the red flags are usually obvious. Look at his or her track record. Look at the realism of his or her vision. Look at his or her ability to model and properly account for risk, competitive threat, etc. If you can pinpoint a critical dependency or risk to the business that he or she can't, that's a bad sign.<p>4) They should be willing and able to bust their asses and constantly learn. They should not expect to kick back and let you build the product for them. Sort of hard to suss this out in conversation alone, but track record evaluation -- and careful probing about the vision -- should tease out some hints in this direction. A sense of entitlement, or of learned helplessness, usually outs itself under polite conversational fire.