One of the most effective ways I've found to validate is to put up a landing page with a brief overview of your offering and, explain that you'll be launching soon and ask the user to enter their email address into a Mailchimp-linked signup box.

Then spend $30 on super-targeted Facebook/Google ads for your niche and make sure you ad links to the landing page.

If 25-30% of visitors enter their email address then you're onto something.