Product/market fit can be looked as being in the right market with a product that satisfies the needs of people in that market.
You know when there is product/market fit and when there is no product market fit.
So when a product is growing at a fast pace and is getting promoted by the users of the product, we can conclude that there is produc/marlet fit. But when none of this seems to happen we know that the product is not fit for a particular market.
When you get feedback from your customers about the product and if minimum 40% customers say that they would be disappointed your product wasn’t their than your product is a fit for the market.