How do you know what to test at the right time on a product?

You have a hypothesis of a new feature that would improve x on your product and you don't have any data backing up your decision as this feature is new territory. How do you know this hypothesis is better to test now than other hypothesis, and what is the best way to test it without spending any time coding or even designing it?

Do you put a fake button to see if people are interested in this? Do you sent an email and see how many people interact to see what is upcoming? Do you make people test a website that already has this to see how they react?

I feel like doing user interviews for qualitative data in this case can be misleading because if 6 participants say that they would use the feature it doesn't mean that the rest of the people will.

So the question is, how do you know what to test at the right time so you can move fast?