Stamps have been marked on the back by experts for a very long time. It's more of a thing in Europe than the US, where we've had certificates for nearly as long.
German and Spanish expertisers will mark forgeries - usually with the word "false" in whatever language they use.
But most of Europe is moving away from marking the stamps on the back, and towards certificates. Modern ones have a photo of the item on the certificate, which isn't perfect.
The question that arises is what to do with an item that is determined to be fake or altered, but later can be proven to be good? It's a tricky thing no matter how you look at it.
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