A [custom meta box][1] with a checkbox field would probably work best, but the easiest solution I can think of is this:

 1. Create an "Edited" category.
 1. Make sure you are using `post_class()` on the wrapper element of each article/post/page.*
 1. Add a CSS rule that looks for `.category-edited` class on the post wrapper (or .category-{ID}, which is safer if the "edited" slug ever changes).

The `post_class()` function (read about it [on the Codex][2]) is a template tag used in theme files. It's put on the wrapping element for each post in a loop. It can be on any HTML element, but common ones are `div`, `article` (HTML5 only), and `li`. Using the class looks like this:

    <article <?php post_class(); ?>>
        <!-- stuff like the_title() and the_content() goes here. -->
    </article>

`Post_class()` then generates something like "class='post post-23 category-events'".

  [1]: http://codex.wordpress.org/Function_Reference/add_meta_box
  [2]: http://codex.wordpress.org/Function_Reference/post_class