What are your key indicators for achieving 'Product-Market Fit'?

For us it's mostly the daily active users.

Your product is simple enough + user payment rate, so that you can make better judgments

High retention rate.

I think it's the type of concept that you know when it's happening. Customers buying your product/service as fast as you can make it and rushing to hire some people to support growth. Otherwise, most common definition is when 40% of surveyed customers say that they would be “very disappointed” if they could no longer access your product or service