I just undertook this exercise last week. I setup a 'landing page' on QuickMVP.com to collect emails of those interested in my app. I drove paid traffic to the page via AdWords and Facebook Ads and via my friend/colleague network.
See getsparqed.com for my page.
The thinking behind the setup is true customer intent is measure by a willingness to pay (or in my case a willingness to enter email address). I had a 30% dropoff from people who indicated they wanted to download my (yet to be created) app and those who entered their address.