You can include some code in the WindowSelectionChange event. The Sel
parameter that is passed to the event code gives you the current selection.
You can work out whether the selection is in a header or footer by checking
the StoryType property.

This article gives you an example of the kind of thing you can do with this
event.

How can I prevent users from editing the header of a document in Word 2000
or higher?
http://www.word.mvps.org/FAQs/Customization/ProtectWord2000PlusHeader.htm

That will at least tell you whether the user might have edited the headers &
footers - if the user never goes there, he obviously didn't edit anything
there, at least while your add-in was connected.

However, if this is not good enough for you, you may just have to do a
comparison between what the headers contain and what they should contain.