There are many ways to build initial product/service in which it will validate your "leap of faith."
How many users do you expect that they are going to buy/use your product/service (Let's say in the first year of launch)
How might frequent single user use your product/service during the product/service lifetime?
How many competitors are in the same market you are targeting?
How much revenue do you think you will generate from each customer ? Revenue does not mean it is net profit still there are expenses we need to take away
How much does it cost to provide the customer the product/service in term of time & money?
How much money do you need to start your business idea and produce/provide the first MVP?
When business is up and running , does it require from the founder to be full-time or can founder delegate most of his tasks without affecting the business?
How scalable is your Idea? can it grow fast or it's a slow growing business?