Quickly create a landing page using something like Unbounce then drive some traffic to it ie. spend $100 on AdWords.
Offer something like free access to your product or service for the 1st 100 signups. If you get traction, you'll know you've got something. Then you can use those initial signups as a beta group.
I'm in a startup accelerator, and I can attest validation is one of the most important components they drill into us. Do whatever you can get in front of people and at the very least get people to say 'yes I want to use your platform', the more passionate the better, but even better than that is get signed contracts/deals before launch, or people who are passionate or excited about your app.
