1) Tell everyone about it and gather opinions and criticisms. No one person should dissuade you or boost your confidence in the idea, but a broad spectrum of advice will open your eyes to problems or opportunities you won't have thought of
2) Set up a basic landing page (try unbounce.com or strikingly.com) and set up a low cost Adwords campaign to drive some traffic to it. If you're getting people to sign up for more info that's a good sign, and also gives you potential customers to speak to and learn more
3) Depending on the target market find somewhere you can engage with a bunch of target customers, be that a physical location or a niche forum maybe. Similar to #1 you're just looking for plenty of feedback from the right people

This would give you a good guide on the "demand" side of things. The next step would be to work out if it's financially viable with a big enough potential revenue stream to make it worth the cost and effort of running it...