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()`][2] 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).


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