What’s the best way to find product market fit?

Eric Ries (Author, The Lean Startup)
The Lean Startup: How Today’s Entrepreneurs Use Continuous Innovation to Create Radically Successful Businesses

Marc Andreessen, the legendary entrepreneur and investor and one of the fathers of the World Wide Web, coined the term product/market fit to describe the moment when a startup finally finds a widespread set of customers that resonate with its product (p.219)