Hitting ⌘ + Z Undoes the last action, and ⌘ + ⇧ + Z Redoes it.
In order to see what exactly you are about to Undo/Redo, you can click on "Edit" on the top of the screen.
Question:
What is the keyboard shortcut to find out what exactly you are about to Undo/Redo?
Best Answer
I don't think there is a shortcut to see what Undo would do; if there is one I don't know about it.
The workaround I'd suggest would be fn+ctrl+f2 (focuses the menu bar), then e (selects the Edit menu) and then ↓ (opens the menu).