Dear Dale - i could not find RhinoDoc.UndoActive nor RhinoDoc.RedoActive in RhinoCommon.
As a workaround i use:
Rhino.Commands.Command.BeginCommand += OnBeginCommand; Rhino.Commands.Command.EndCommand += OnEndCommand;
and check CommandEventArgs CommandEnglishName == “Redo” or “Undo” and then set a boolean flag - to replace above Properties.
Works fines so fare - or is there any argument against this approach ?