There should never be nodes that are inaccessible, so if I understood this correctly, this sounds like a bug, although I was not able to reproduce that behavior. If you can get the control into a state where there is no scrollbar but there are nodes that cannot be accessed, you should report it as a bug.

To answer your question, you can determine whether there is a vertical scrollbar currently being displayed in the following manner:

private bool? HasVerticalScrollBar( UltraTree treeControl )
{
    UIElement controlElement = treeControl != null ? treeControl.UIElement : null;
    if ( controlElement != null )
    {
        foreach( UIElement element in controlElement.ChildElements )
        {
            Infragistics.Win.UltraWinScrollBar.ScrollBarUIElement scrollBarElement =
                element as Infragistics.Win.UltraWinScrollBar.ScrollBarUIElement;

            if ( scrollBarElement != null && scrollBarElement.Orientation == Orientation.Vertical )
                return true;
        }

        return false;
    }

    return null;
}

I'm also having the same problem. My wintree will look like this.

 There are 2 more nodes before xls node. I have set the Scrollable property to 'ShowIfNeeded'.

If i delete the node after xls, the scrollbar is not visible.

If i change the Scrollable property to 'Show', after deleting the nodes, i move the mouse scrollbutton, then the scroll bar gets disabled. We are not able to view few of the top nodes(nodes before xls) .

Is there any way to show scrollbar?

http://community.infragistics.com/forums/p/57152/291903.aspx#291903