I have been seeking a simple question I can ask prospective business owners to judge how far along they are in their business - and whether they understand where they need to be to come into the incubator or get a bank loan.
Jason Cohen on his "A Smart Bear" blog states what he looks for: 10 customers. 10 people willing to give you money for your product/service (even if it is not ready).
It is a useful question. If the prospective business owner can't show anyone is interested in paying for his product - then why go forward?
Since I am not dealing only with software startups, I might be able to weaken the requirement a bit, but the principle ("Who is buying your product?") is so critical to success that it could function as a single question to get started.
So - who is buying your product?