SCSS

by allcaps

HTML

<p>
  <a href="/">Internal relative</a>
</p>
<p>
  <a href="/media/foobar">Internal document</a>
</p>
<p>
  <a href="http://fourdigits.nl">http://fourdigits.nl</a>
</p>
<p>
  <a href="https://fourdigits.nl">https://fourdigits.nl</a>
</p>

SCSS

a {
  &[href^='/media/']:after {
    content: " (internal media)";
    color: red;
  }
  &[href^='http']:after {
    content: " (external link http(s))";   
    color: green;
  }
}