How to validate that the problem is a Tier 1 problem?

Hey,
I want to build a product that could solve a potential problem.
I plan to conduct interviews with potential clients. I read several articles on the manner, including this one - https://www.startupgrind.com/blog/the-startup-framework-to-validate-your-idea-before-you-spend-1/, and I wonder what kind of questions can I ask to understand if the problem is real for the potential user (AKA, would pay for it) or a nice to have?