At a high level: you have found product market fit when you can repeatably acquire customers for a lower cost than what they are worth to you. This is a simple business statement. Are your costs lower than your revenues? Are you profitable ?
For true, lasting, durable, product-market fit, you need:

In the following article, Marc Andreessen makes the market is the first criteria of startup success, underlining :
Product/market fit means being in a good market with a product that can satisfy that market.
<aside> 💭 “The only thing that matters is getting to it.” implies at the early stage of the startup you should focus solely on assessing, reviewing and validating your PMF and not on other operations (recruiting, scaling sales too early, non essential and costly engineering …)
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