More questions to ask: What is their source of funding (VC, bootstrapped with founders' money, etc.)? What is their business plan, and at what point do they anticipate being profitable? If they're not profitable from the beginning, how long do they think they can run the company on the funds they have? Would they expect you, as a co-owner, to help fund the company? Do the founders have any prior business experience? Have they researched the market they're entering?

If you don't know how the business works, there's no way for you to evaluate whether their equity is worth anything at all.