Talk with people to develop the idea as far as you can, but you don't truly know until people pay you. So you have to deliver a product (unless you're lucky enough to get pre-sales but even then you've likely invested something in a prototype) which is that point your stomachs tightens up as it requires some amount of money to develop this product. Aka the moment you find out if you really believe in this idea, enough to risk it.
Whether it's server costs, building a physical prototype, buying a data set, whatever it is.
Someone buys it, cool. Then don't spend anymore money until you've repeated the sale several times to validate you didn't just find this one unique customer.
A few would-be entrepreneurs have asked me for advice (though I'm not sure why) and are looking for that whole reverse prescription to success through how other people did it, which is a fallacy - there's only your path. I've noticed that they were stuck in this idea phase and it always came down to that moment of putting your money where your mouth is. I would just tell them either you believe in your idea enough to pay the $5,000 to fabricate the metal for your prototype, or $500 for the up-to-date information for your real estate developer app, or whatever the thing is that is your next step, or you don't because you don't really believe in the idea enough to take the risk. Which is fine, just recognize that and keep searching.