How do you know what KM has had done OP
By looking at her?! 
I mean, when my roots magically change colour from brown to blonde, I don't need to officially tell people I've had my hair done. They can see for themselves.
The giveaway with Kate is that she stops having Botox when she's pregnant. Lines appear on her forehead and then magically disappear again.
She has had significantly more work in recent years though, as the fuller face is new.
Absolutely no criticism here - I'd do the same in her shoes. It's been well done - it's subtle enough to still be fooling people, apparently- and she looks fantastic.