I couldn’t find a very easy way to do this either, so I just copied what WordPress was already doing in get_next_posts_link().
<?php
if ( !$max_page ) $max_page = $wp_query->max_num_pages;
if ( !$paged ) $paged = 1;
$nextpage = intval($paged) + 1;
if ( !is_single() && ( $nextpage <= $max_page ) ) {
?>
That block evaluates to true if there ARE more posts to show — so in here I have my call to next_posts_link(‘More’). If you want to do something when there aren’t any more posts to show, just put in an “else” clause after the last “if”.