If Undo is active, its menu text is changed to (Undo[command]); if it's not possible for CK to Undo, the menu text reads (Can't Undo) and is dimmed. Selecting (Undo) changes the menu text to (Redo[command]).
If you want to Undo, do it quick! Clicking on controls, activating windows, and many menu commands turn off (Undo).