Hooking Undo / RegisterUndo

I have a simple goal: to determine when a file is being modified in memory - i.e. a change now, when the modification in memory is made, not when the file is saved later (that I can do with the SaveAssetsProcessor thank you).

Since unity has a way to register modifications for undo, I could catch most occurrences of modification by hooking the RegisterUndo methods.

How?