Is there a system that you can implement to find product market fit? This is a question that I am exploring right now and will publish my findings soon enough.
Until then, there is only one answer that I am confident enough to say is a tautology. Talk to users who enter your product and talk to user who exit your product.
Entry points
Talking to users who just entered your product allow you to understand the real motivation someone has for selecting your product. Often, the reason, or set of reasons why someone chooses to use your product is much different from what you think. The best way to understand the circumstances that motivated someone to use your product is understanding the story that got them there.
Exit points
Talking to users who left your product are also great sources of insight. Why? because it helps you understand the real problems that your product must solve. For example, I used to use Bear Notes on my mobile phone. Recently, I deleted the app, because Notion made some improvements to their mobile app. Point being, I was using Bear notes as a work around. If the team had spoken to me upon exit, they would have would found out that I used their product for quick notes on the go, usually jotting down notes from listening to podcasts. As a result of talking to users who left your product, you will be able to identify the functional and emotional features / benefits that will allow you to increase switching costs.
What if you don’t have a product yet?
In this case the entry points that you speak to should products you identify as being what your users will stop doing, once they find your product. For example, I started using Amazon Prime Whole Foods Delivery, and as result I stopped going to Trader Joes. As for exit points you should speak to users who recently stopped using whatever you would deem to be “competition”.