Apart from having a landing page and trying to get it ranked for certain terms, I don't think you can validate an idea without being able to show/tell people what it will look like. The quality of the product will be judged upon what it looks like, if I made a 1995's style white page with just two form fields and a 'submit' button, that couldn't be used to validate the idea with.
In my mostly-unqualified-to-comment-opinion, that is.