Everyone talks about product market fit, but you should really first be talking about problem market fit.
You can boil problem market fit into three things.
1) Identifying a Real Problem: Confirm the problem is something people genuinely struggle with and are actively seeking solutions for.
2) Target Market Validation: Who is experiencing this problem most acutely.
3) Willingness to Pay: Are they willing to pay for a solution to their problem.
How do you find problem market fit?
Look at possible existing competitors. Research current market trends and drivers. And most importantly, talk to a lot of people.
Finding problem market fit will ultimate save you a lot of money and time.
Because if you can't find it, then don't build a product and spend time and money trying to acquire customers for a product that doesn't have real utility.
A great outcome of problem problem market fit is you're going to build a minimal viable audience along the way. Or at least you should be.
Then comes building a minimal viable product.