Rahul Vohra, the founder of Superhuman, took a rigorous approach to finding and measuring product market fit for his startup. He asked customers a simple question - "How would you feel if you could no longer use our product?" - and measured the percentage who said "very disappointed".
The key to this approach is to segment the responses based on customer types to identify exactly which customers are most satisfied. Once you have identified a target customer, focus on what is holding them back from responding "very disappointed". These insights will drive your product roadmap and focus.
This process of segmentation, identifying feedback to focus on where to improve, and continuing to optimize the product's existing strengths can be repeated to reach product-market fit and turn your passive users into superfans.