Matt,Here is some code:
//Add this attribute to the toc element
itemRollOver="setLastTreeItem(event)"
//Add these import
import mx.events.ListEvent;
import com.esri.maplib.tocClasses.TocItem;
//Add this var
public var lastTreeItem:ListEvent;
//Add this new function
public function setLastTreeItem(event:ListEvent):void
{
lastTreeItem = event;
}
//Then you can add something like this to your function to get a TocItem
var item:TocItem = TocItem(lastTreeItem.itemRenderer.data);
from the item you can get the label or check for isGroupLayer.