What's the best way to validate a SaaS idea?

I think you can start in a lean way and build a landing page for your SaaS idea as if it is an actual product and start to post it to some websites that features beta or demo apps like... betalist.com... for example and may be drive some targeted traffic by paid Google or Facebook ads.

Attempt to sell it based on your understanding of the problem and what you imagine the outline of the solution to look like. If you can't successfully close sales on air, actually having the product available doesn't solve that problem as much as you'd think it does.