check if vertical scrollbar is present

Is there a way to check if a vertical scrollbar is present?
The purpose is to place automatically a top-of-page link if the scrollbar is present - if no scrollbar is present no link will be placed.

Tx

That works in IE (not tested in NS):


<script>
pg=document.body;
if (pg.scrollHeight>pg.clientHeight) document.write('top of page'.link('#'))
</script>