Does anyone here use blinkist and can help me validate an idea?

I'm thinking of an app that would help users of shortcut-apps like blinkist/storyshots and I'm at the point where I am thinking of how to take it beyond just an idea.

What are some ideas for validating the concept without going to design & build?

How do I find these types of users and people, and is it as simple as putting together a google form to collect some sentiment?

How do you balance revealing too much about your idea, but still collect accurate insights?

Thank you