How can I find changes before saving a file?

[PL]

If I have edited a file and want to see what changes I have made, how
can I do that? I can of course use "Ctrl-x u" to undo the changes
one-by-one, but that's not really what I want to do. Then I would have
to re-do the un-doings to get the file back...

Another way is to copy the file on disk, save the file, and compare
with the copy. But that means I have to save the file. I want to be
able to find the edits *before* I save.

Any ideas, anyone?

Rgds
Peter