WordPress: check if the comment author is the post author.

gistfile1.php

<?php
    /**
     * By default, WordPress add a .bypostauthor class on comments
     * I prefer echo something to have a better control in CSS.
     * Exemple: http://codepen.io/Creaticode/full/ijExL
     */
    global $post;

    if ( $comment->user_id === $post->post_author ) {
        echo '<span class="is-author">Author</span>';
    }
?>