How DO you test the desirability of an idea or startup?

You may have heard us talking about validating an idea, and part of that is to test the desirability of the idea. Desirability Validation is used to de-risk an idea and test the solution, without building it first. This helps to save you time, effort and money.
It also helps to give you confidence that the problem is worth solving; and that people will pay for the solution.
But… how do you actually do that?