Visual prototypes can go a long way for validation. I recently validated a product Idea by:
- Sharing the prototypes with everyone I could get to meet IRL, asking for feedback and what they thought about the idea.
- Created a landing page to gather signups and show a bit about the product.
- Posted visual designs on dribbble, HN, reddit, twitter, etc (to your relevant target audience) - this feedback was invaluable as it comes from people who are not attached to you in any way. You may also get lots of signups this way.
- Promote the landing page in relevant communities, be straight forward and say you are validating your idea - even more signups.
I was still building my product in the background, but reaching over 900+ signups in a few weeks for a product that was not even out there counted as idea validation with just a set of visual prototypes.