What we should have done instead was to focus on the problem and market, then search for the solution. This common mistake is why I prefer “Market Product Fit” over the terminology “Product Market Fit.” 

the real problem is something experienced within your market and by your audience, not something that lives within your product.

Category. What category of products does the customer put you in?
Who. Who is the target audience within the category? There are always multiple personas within a single category, so this breaks it down further. 
Problems. What problems does your target audience have related to the category?
Motivations. What are the motivations behind those problems? Why are those problems important to your target audience?

Core Value Prop. What was the core value prop of the product? How did it tie to the core problem?
Hook. How could the core value prop be expressed in the simplest terms?
Time To Value. How quickly could we get the target audience to experience value.
Stickiness. How and why will customers stick around? What are the natural retention mechanisms of the product?

In practice, the search for market product fit is never a straight line. Instead, it happens over multiple cycles of iteration. You start with a market, build an initial version of the product, look at who actually gets value from the product, then redefine the market and redefine the product.

Market Product Fit is not binary. It’s also not a single point in time.

A better way to think about Market Product Fit is on a spectrum of weak to strong. 

To get a qualitative understanding, my preference is to use Net Promoter Score (NPS). If you are truly solving the audience’s problem, then they should be willing to recommend your product to a friend. 

The biggest downside with qualitative information (vs quantitative) is that it carries a higher probability for generating a false positive result, so take it with a grain of salt.

There are two quantitative measures to understand Market Product Fit: Retention Curves and Direct Traffic. 

for 80% of B2C and B2B products out there, flat retention curves is what you are looking for.

Direct traffic is typically the result of word of mouth. If you are truly solving an audience's problem, they tend to tell friends. It might not be a lot of direct traffic, but there should be some.

“Product market fit doesn't feel like vague idle interest. It doesn't feel like a glimmer of hope from some earlier conversation. It doesn't feel like a trickle of people signing up. It really feels like everything in your business has gone totally haywire. There's a big rush of adrenaline from customers starting to adopt it and ripping it out of your hands. It feels like the market is dragging you forward.

Start with the market (problem), then the product (solution). Not the other way around. 
Define your market hypothesis using Category, Who, Problems, Motivations with most of your work going into Problems/Motivations. 
Define your product hypothesis using Core Value Prop, Hook, Time To Value, Stickiness.
Think of Market Product fit as a cycle. 
Understand that Market Product fit is not binary. Instead, it’s a spectrum of weak to strong.
To understand if you have Market Product Fit, combine the qualitative, quantitative and intuitive indicators.