A lot has been written about how to tell when you have product-market-fit. Some of the classic ways is to run a survey asking how people would feel if they could not use your product anymore. As per Sean Ellis, if over 40% of your customers say they would be very disappointed if they could not user your product, you have fit. You could also measure your NPS score – though interpretation of that is up in the air. Also, as with all surveys, make sure you cover all your channels. Each channel will have some sort of bias.
My take on how to tell if you have fit, is that if you have fit, things just seem a bit easier. Acquisition is easier, users are spreading the word about your product, partnerships come easier. I realize this isn’t the answer that most people want, but I feel it’s very true.
I also find that it’s very easy to answer the question, “Why do customers come to you?” in a clear and concise way that is backed up by user interviews and behavioral data.