[neom]

The general rule of thumb for PMF is if you're a business and someone is paying you for your product, your product is fitting into that market. So your first questions to potential customers (who you trust/have vetted) should be: If I built this, would you pay for it? Why? How much? Why? Ok, I'm going to build this and it will cost X, will you be my first customer please (/and sign an LOI/contract)?

More generally: this a very big and complicated question, PMF can feel very different in every business or org, and PMF might differ between Oxfam and Microsoft -for example. Either way, it's generally always distilled down to: the value exchange makes sense to everyone involved; so figure out how to validate that in your ideas.