How to determine if ToolWindows are docked with tabs?

I want to hide the titlebar of my ToolWindows if they are docked in such a way that tabs are displaying - i.e. when two or more toolwindows are grouped together. I think it is redundant to show the titlebar since the tab already displays an appropriate name.

I am using the DockManager_ToolWindowDragged even to iterate through all the ToolWindows in order to set the HasTitleBar property but I cannot find any way to determine if the toolwindow is grouped into tabs with other toolwindows - how do I determine this?

Thanks,
David