use DOM...
some pseudocode to get you started (for onsubmit of the add to cart button)
Code:
var selall = boolean(true);
var sel = document.getElementsByTagName("select");
for(i=0;i<sel.length;i++)
{
if(sel[i].value == "")
{
selall = false;
break;
}
}
if(sellall)
add to cart
else
alert user that they need to select option(s)