Looking for a hook for post.php

You can add your code to the init action hook and check global $pagenow variable:

add_action( 'init', 'wpse8170_check_from_var' );
function wpse8170_check_from_var() {
    global $pagenow;

    if ( 'post.php' != $pagenow || ! isset( $_GET['post'] ) || ! isset( $_GET['from'] ) || 1 != (int) $_GET['from'] ) {
        return;
    }

    update_post_meta( (int) $_GET['post'], 'EDITED', 'true' );
}