How to add a "viewed" icon to pages that are visited
In my opinion using a cookie could be a nice way to accomplish that need. (Maybe even the only way).   A quick search came up with the user_cookie_save function >> https://api.drupal.org/api/drupal/modules!user!user.module/function/user_cookie_save/7

Use it to save the cookie with the video URL or another identifier and read that cookie when loading the page and set the viewed flag.