What stage are these founders in? Have they succeeded, are they on their way there, are they just in a the planning stage? Different questions apply to different stages.
How much money do you think you'll make from this (to see how they evaluate their chances and worth), do you want to sell or remain and make a salary (to see what they are building the company for - longevity or a quick flip, not that either are bad), and what do you see your retirement like?