At my startup [0] it's still early days, but we're trying to use the Superhuman "engine" [1] for testing PMF. Basically this is a survey where you ask your users how disappointed they'd be if they couldn't use your product anymore, and if enough say "very disappointed", then you're on to something. By no means is it the only/best measure of PMF, but it's a data point.
1: https://firstround.com/review/how-superhuman-built-an-engine...