How you test and measure for product-market fit can vary from one business to the next. The most important distinction as to how it varies is whether your company is a network effects business or a non-network effects business. Here's a simple framework I use...

If building a network effects startup, the way to identify and test for product-market fit is by looking for users that are trying to expand the network for you. It's a revealed behavior. A few examples of network expansion behaviors...

The obvious being that they invite others to join. In the earliest days, don't provide invitation functionality (e.g. for the first 1,000 users). Then build invite functionality and see who latches onto it. The inviters have revealed themselves as your ideal customer profile...

Spend time digging into those users to understand what makes them unique relative to the non-inviters. Secondary and tertiary examples of network expanders would be people that comment or produce new content into the network vs just the lurkers that never create...

From those revealed behaviors you've been given a gift: an audience that has naturally exposed the customer personas to you. Your job is to now get to know each of those audiences more deeply and then amend the product and customer acquisition roadmaps based on what you learn...

For non-network effects businesses, the approach is different. You don't have network expanders to study. Instead, a non-network business tends to have a core transaction that you can instead observe. For example, on an app like Calm the transaction can be a subscription...

Similarly, you can study users that segment themselves through their actions (either they subscribe or don't). But before you go down the path of building new features that require an explicit commitment from the customer, you want to test the concept to validate its potential...

This is known as the sell-design-build method i.e. you want to test "selling" the product to the customer before you design and build it. What you want to do is mock up a prototype and have potential customers go through it in a customer research environment...

Once they explore it you may eventually ask a question like "Would you be willing to subscribe if we built this?" Many will say "yes" but there are false positives. To cut through that you surprise them with a take away e.g. put a credit card form in front of them on the spot...

This will surprise them. You WANT TO SURPRISE THEM! That's when the truth comes out. If they don't want to subscribe on the spot then you have cut through the noise of them casually saying "yes" to the prototype. The magic is in drilling down on those...

that initially said "yes" but then hesitated once you forced a commitment. That's when you push to understand why they wouldn't actually commit. That's when you truly start down the path to distinguishing if you have PMF, and for who you may or may not have PMF with.