On my NTabControl I've specified

hasClose = true and showCloseOnEachTab = true

I also have registered to receive a ClosingTab event.

How can I code the ClosingTab event to determine which tab is being closed?

Setting the Cancel parameter to true does stop the closing of the tab but its not clear how to know which tab it is.  I'm on the 11.12.14.12 build.

Within the event you'll receive a DocumentCancelEventArgs. This event handler has a property "NUIDocument Document" which you can ask for the Text property to determine the name of the document or the Client property to get access to the hosted control.