- Take your idea, build a simple UI (not the app at all, just the UI)
- Take that UI, and get a sign up modal on it
- Take that and run ads that point to it.
- See if you get enough conversions that justify you actually building the thing. What's that number for you? Dunno, but you'll learn quickly. And then you'll have a small mailing list of beta testers.
You can toil away and build a thing, or you can find an audience, and then build a thing that those people will pay for.
Everything will follow from that.
(or if you are going for a 'global scale' leveraged play -- you need to get users, cover the costs of the platform, and then get VC backing to figure out your monetization strategy)