Drupal 7 + entity translation + path-prefix language URLs.
The vast majority of the site is default language only. Certain pages are translated into multiple languages. When the user reaches one of these pages, and switches to a non-default language, url paths all now point to path-prefixed pages.
I'm using language fallback, so they'll see the default language content, but at a url with the language code as a path prefix.
Preferably, I'll get Drupal to check first to see if there is actually an entity-translated page on the other end of the request, and, if there is, pass it on through, and if not, re-write the URL back to the default-language (no path prefix) version. So as to avoid SEO/duplicate content issues and be a mildly better user experience.
My question: how do I achieve this?