Just have one more link or div that says "No more posts" below your all the post., and on ajax success funtion you can show this.
Something like,
<div id="no-more-posts" style="display:none">
No more posts to show.
</div>
success:function(html){
$('#show_more_main'+ID).remove();
$('.posts').append(html);
$("#no-more-posts").fadeIn();
}
-KR