How do you filter out polite encouragement from people who will actually pay?

When starting out it’s so easy to mistake a long feature request list for genuine interest.

A user tells you they’ll sign up as soon as you add one more thing, so you waste two weeks building it, and then they still don't use it.

Now I try not to trust anything unless someone gives upfront feedback on a paid tier or actually logs in without me sending a reminder email.

How do you test if someone actually needs what you're making vs just being nice to you?