by server side
you should add "runat=server" attribute with li
<ul><li id="liFamily" runat="server" class="current"><a href="">By Family</a></li></ul>
Now you can access this on Server Side
if you want to do it on client side
you should use javascript function at the last of the page
and you should khow about current menu then you can add attribute..
make function as globla; and call function on each page at the last
with current menu.
function setmnu(mnuid)
{
var mnu=document.getElementById(mnuid);
mnu.className="current";
}
setmnu('liFamily');
ok