See What Redo/Undo Will Do on Mac 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?
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 &darr; (opens the menu).