A general business person is not much use to a technical startup early on. To paraphrase Peter Drucker, every business has only two core functions: innovation, or marketing. So any "business" co-founder had better be awesome at marketing/sales (those are related, but not the same), preferably with domain knowledge and people-access in the market of interest that's difficult for outsiders to get. Typically, you could evaluate those skills by outputs: reading a marketing document, seeing a presentation, and drilling them with questions. Just like with programming jobs, titles don't mean much; there's no shortage of schmoozers and BS'ers who can't actually do marketing well, especially in the fluid, fast context of a startup.

Beyond those business skills, you'd likely want the usual requirements of any co-founder: integrity, and a record of getting things done.