Check if comment was modified

You should be able to do this fairly simply by hooking to the edit_comment action and saving the modified date using update_comment_meta:

add_action('edit_comment', 'my_comment_modified_date', 10, 2);
function my_comment_modified_date($comment_ID, $data) {
    update_comment_meta($comment_ID, 'modified_date', time());
}