Edit in JSFiddle

$('.content a[href^=http]').each(function(){
    $(this).not('[href*="'+location.hostname+'"]').append(' [External Link]');
});
$('.dummylink').click(function() {
   return false; 
});

.content {
    width:300px;
    margin:auto;
}

<div class="content">
a href="dummypage"<br>
<a href="dummypage" class="dummylink">Lorem ipsum</a><br>
a href="http://google.com/"<br>
<a href="http://google.com/" class="dummylink">Duis aute</a>
</div>