I've written before about Bootstrapping, which is an important principle for business founders to understand so that they don't get blinded by their vision and "bet the house". 

Paying customers are the only metric of business success, and I advocate spending as little as possible prior to securing your initial orders as this prevents you from backing the wrong horse. There is no marketing research like asking a potential customer to buy.

But Bootstrapping alone does not provide enough of a toolset to discover your market. That is where the Lean Startup comes in.

Hypotheses
Hypothesis Testing
Business Model Canvas
Get out of the building
Pivot