Close Window Link Only Visible in Popup

Please help! Does anyone know how to create a text link "Close Window" that will appear on a popup window. The trick is, I need it to appear only if the window is a popup window not if it is the parent window. Please help! Thanks.

Bill

<script type="text/javascript">
if (opener) document.write('<a href="#" onclick="window.close()">Close Window</a>');
</script>

Thanks Glenn, this works great!!!