How do I test product-market fit?

How do I test product-market fit is mainly about determining whether a defined market consistently adopts, retains, recommends, and pays for a product. A reliable approach is to measure retention, repeat usage, renewal, referrals, satisfaction, organic demand, sales efficiency, and customer dependence on the product. For example, for ecommerce, strong fit may appear through conversion, low returns, positive reviews, repeat purchases, referrals, and stable acquisition economics. The main caution is that early sales driven by discounts, novelty, personal relationships, or unsustainable advertising do not prove fit. The final output should provide a segment-level conclusion about where the product creates durable value and where it still fails.