Here is a really good example of how a startup validated their product without building it.
The startup is called SendWithUS.
They wanted to:
a) gauge interest in a transactional email service, and
b) measure what features mattered the most to customers.
To accomplish this they decided to build a website that seemingly offered many features and measured which ones customers were most interested in. Once they had an answer, they’d build an MVP focusing on those features. Easy, Smart and Lean :)
See the full case study here: http://goo.gl/k1UOGP
Hope this helps.
Good luck building your MVP and the startup!
PS. Let me know if you need assistance on applying this method to your idea.