Wordpress - How to determine if theres a next page

You can use get_previous_posts_link and get_next_posts_link to determine if they exists like this:

$prev_link = get_previous_posts_link(__('« Older Entries'));
$next_link = get_next_posts_link(__('Newer Entries »'));
// as suggested in comments
if ($prev_link || $next_link) {
  echo '<ul class="navigation">';
  if ($prev_link){
    echo '<li>'.$prev_link .'</li>';
  }
  if ($next_link){
    echo '<li>'.$next_link .'</li>';
  }
  echo '</ul>';
}

Hope This Helps

I wrote this up a while ago, but should still be valid:

http://www.ericmmartin.com/conditional-pagepost-navigation-links-in-wordpress-redux/

You can add the following function to your functions.php file:

/**
 * If more than one page exists, return TRUE.
 */
function show_posts_nav() {
    global $wp_query;
    return ($wp_query->max_num_pages > 1);
}

The update your code to:

<?php if (show_posts_nav()) : ?>
<div class="navigation">
    <div class="next-posts"><?php next_posts_link('&laquo; Older Entries') ?></div>
    <div class="prev-posts"><?php previous_posts_link('Newer Entries &raquo;') ?></div>
</div>
<?php endif; ?>

the best solution is checking $wp_query->max_num_pages, but you also can use:

<?php
if(paginate_links()) {
...
}