I'm a journalist with a background in science, and have a solid record for it. But I am a wee bit lost when it comes to fundraising, sales and suchlike, at least partly because of the 'wall' maintained between marketing and journalism in most places. Now I need to work with someone who understands the concept and can still work within it. Ergo, the question...
The advice above is solid re: having a proven track record. What I'd add is that the cofounder relationship is also very personal. Bootstrapping a startup can be incredibly challenging and stressful, you want to be as certain as possible that whoever you involve is someone with whom you're ready to share the metaphorical foxhole. The softer, personal questions aren't easy things to assess, but here are some questions I've used to do so in the past:
Startup life isn't all doom and gloom, but there will be bad days and you need to be prepared. You don't have to take my word for it though -- Ben Horowitz (of Andreesen Horowitz) wrote an excellent book on the subject called The Hard Thing About Hard Things. It may be worth a look before you consider marriage to a co-founder.
Why is it that you feel your skills are validated to a potential cofounder? Look for those similar qualities.
I suggest putting a list of skills together that you believe is needed to complement your own, then look for validation against those skills by using the list above or brainstorming similar.