yo uso esto, a ver si os vale.
function isTopLevelItem(treeW, item){
var num_ramas_princ = treeW.topLevelItemCount;
var principal = false;
for( var i = 0; (i < num_ramas_princ) && (principal==false); i++){
item_prin = treeW.topLevelItem(i);
if(item_prin == item){
principal = true;
}
}
return principal
}