How to validate product market fit before building it.

Depends on your product, but what I would do is build a few landing pages that succinctly and clearly describes your product and the main value proposition. Each landing page can test a different version or feature. Then create Facebook ads that correspond to each individual page in messaging, then drive traffic via ads and measure conversion to email sign-ups. If you have the budget, you can run variations of the ads to different targeting groups to understand your how it affects your key KPI's. At the end of the day, you can gauge interest, size of the market, and early glimpse and unit economics.