You can verify it by checking the authors handwriting, the color of their ink and how the tip of the pen has indented the paper. That is difficult to spoof with AI.

So, what you're saying is you want someone to make a machine that can clone their handwriting.

Perfectly cloning someones handwriting so that it is indistinguishable in all circumstances is generally considered not fully possible

The same is true for perfectly cloning your own handwriting.