Customize the Thesis 2.1 Comment Intro Text and use the count feature.

comment_intro_text_count

/* Customise the comment intro if there were comments or not */
function comment_intro_replace($text) {
    global $wp_query;
    $number = (int) count($wp_query->comments_by_type['comment']);
    if ($number == 0)
        $text = '';
    return $text;
}

add_filter('thesis_comments_intro','comment_intro_replace');