It depends on the specific context of what you're trying to build, but I'd say that one of the best ways to initially test if there's customer demand for your idea is to see if people are already trying to solve the problem you've identified through some other means. To give you an example:

Let's say you want to build an app that helps businesses pre-screen candidates for interviewing -- how do businesses solve the problem today? Well, the hiring manager / HR team might do their own research by calling people who know the candidate and asking for feedback. Or the business might hire a headhunting firm to do this for them.

If people are already be paying someone to solve this specific problem for them, it's a good indicator that your idea might be worth something (provided you can come up with a better solution).

However, even if people are not currently paying someone to get their problem solved (but the pain point still exists, and people are still coming up with solutions of their own) it might be worth evaluating to see if you can come up with a solution + a business model that might work.

I hope this is helpful and let me know if you'd like any additional feedback!