The true answer is extremely straightforward - you gain multiple paying customers.
We live in a capitalist economic system, and money is our store of value. Therefore if a customer buys your product, they are clearly demonstrating that you are delivering them value at least equal to the purchase price. Selling the same product multiple times without customization proves you have built something repeatable.
All other ways of measuring product success, whether it be market share, market recognition, analyst awards etc, are irrelevant if you fail to sell the product. One large software company in particular sets goals around market share, but it does this because as a certain % of market share translates to a certain % of market revenue.
Therefore, any company that wishes to minimize product risk, needs to get to market rapidly, with a minimal viable product (MVP), and gain paying customers during the development process. Finding 2-3 customer willing to commit to buy your product and engage during development dramatically reduces product risk. If you can't get multiple customers to commit to buying your product before you start developing, this may be a signal that you are building the wrong thing.
As an example, while at Citrix we developed the Citrix Secure Gateway (CSG), that later morphed into the Access Gateway and quickly became number #1 in the SSL VPN market, and over $100M revenue. CSG was built by a small Citrix skunkworks team in only seven months, and very early in the process Lehman Brothers engaged as our development partner. They actively helped during the development process by reviewing our specs, meeting with the team, detailing their requirements, and committing to using the product.
- No customer(s) who had committed to buying the product on release.
- No focus on delivering the minimum viable product (MVP). Requirements blew out as "must haves" kept on being added.
- No development customer partners.