The Sean Ellis test for measuring PMF
A common technique for determining if you have PMF was invented by Sean Ellis. You survey your beta testers and ask them a simple question: How would you feel if you could no longer use this product? (Very disappointed, Somewhat disappointed, Don't care, No longer relevant because I no longer use it). The general rule of thumb is that is you get 40+% of users saying "Very disappointed", then there's a good chance you have found PMF.