Some code is calling the static method
sun.awt.dnd.SunDragSourceContextPeer.checkDragDropInProgress().

This will signal (via an Exception) if a drag is in progress and
which is the case when DragSource.startDrag(..) has been called.

I don't see any public API that gives you this information.
If so, we should consider adding a (friendlier) standard API that
reports this status.