Tell difference between Undo & Redo on callback

For my editor extension I've tried getting undo/redo working, but have had trouble getting it to play well with Unity's undo/redo system so I've rolled my own and it is almost fully functional.

My issue is t$$anonymous$$s. If control+Z is pressed my gui doesn't get an event for control+Z as I believe t$$anonymous$$s was 'used' by Unity's undo handler. Not$$anonymous$$ng will fire if i ask for both control & Z to be down. If I do just control or just Z my code works fine.

I have subscribed to the UndoRedoPerformed event, but I can't figure out how to tell if an undo or redo was performed. Any ideas?