tell if a list-item is in the visible portion of a ul

tell if a list-item is in the visible portion of a ul

Hi,

I am trying to figure out a way to tell if in a situation like

<ul id="ul1" style="overflow: hidden;width: 200px;">
   <li style="float:left">one</li>
   <li style="float:left">two</li>
   <li style="float:left">three</li>
   <li style="float:left">four<li>
</ul>

so it looks like

one   two   three   four

if I use the left css property of each li to move them left or right, is there a way to tell whether a specific li is visible in the ul or if it is out of its visible area?