The most-used test is Sean Ellis's 40% survey: ask active users how they'd feel if they could no longer use your product, and if more than 40% say "very disappointed," you likely have fit. But one survey isn't proof. The stronger signals are behavioural — a retention curve that flattens instead of decaying to zero, net revenue retention above 100% (customers spending more over time, not less), and a real share of growth coming from organic pull rather than paid. Fit is the moment the market starts pulling the product out of your hands.
The gap that matters isn't the score — it's the decision behind it. We find the truth in your data and your customers: who's "very disappointed" and why, which segment actually has fit, and what the people who leave are really telling you. Then we help you decide — scale, narrow, or fix — and build the thing that follows from it.