void ultraTree1_MouseMove(object sender, MouseEventArgs e)
{
    UltraTree tree = sender as UltraTree;
    UIElement controlElement = tree != null ? tree.UIElement : null;
    if ( controlElement != null )
    {
        UIElement elementAtPoint = controlElement.ElementFromPoint( e.Location );
        if ( elementAtPoint is Infragistics.Win.UltraWinTree.ExpansionIndicatorUIElement )
        {
            //  If you get into this block the mouse is over an expansion indicator.
            //  You can get a reference to the associated UltraTreeNode like this:
            UltraTreeNode node = elementAtPoint.GetContext( typeof(UltraTreeNode) ) as UltraTreeNode;
        }
    }
}