As someone facing just this challenge right now (and not for the first time), I know really well how exciting (and also terrifying) it is to have an idea that just might be the next great business idea.
It's absolutely true that it helps to speak to lots of people before you dive in. Having a great, supportive network of contacts who you trust to respect your ideas and to keep them confidential, as well as to give you honest and constructive feedback, is invaluable. I'd encourage any entrepreneur or intrapreneur to get feedback and ensure there's genuine demand (or buy-in) for what's proposed.
But before you start having any meaningful conversations, you need to do a bit of thinking and planning yourself first, so that when it comes to explaining what you have in mind, you can clearly describe your idea in a compelling way. And importantly, you've also anticipated the possible challenges ahead and thought about how to overcome them, before someone else points them out.
The reverse time-line method that I advocate, and which is described in the linked article, is a simple approach to setting out your idea and 'getting to know it', as it would or could look in a real business environment.
is there a magic formula for knowing whether you’re on to a great thing? Sadly, I think there isn’t... But there are a number of things that you can sensibly do to give yourself the best prospects of success.