There should be some universally recognized icon, like RSS feed icon, which would be on sites offering app versions, and user could press it of he wants to.
Google Play and Apple App Store each have a "badge" graphic consisting of the logo and some white text on a black, rounded rectangle. Maybe Google wouldn't mind if you cut it down to just the Play logo. Apple says not to modify theirs or use the Apple logo alone. I'm surprised Apple doesn't use their normal pencil-paintbrush-ruler icon for the App Store, as the Apple logo alone doesn't necessarily make sense.
It's going that way. Hopefully not some annoying icon featured prominently on every page, but instead a subtle browser-based recognition that the site also has a web app and puts a tiny mobile icon in the browser bar that will launch you to the app's install page.
This functionality is actually built into the Metro side of IE 10/11 on Windows 8. Which sadly means that nobody (especially nobody who reads HN) will ever see it.
