I've revised the translation of the book for a publisher and now they want me to specify the number of pages that was beyond a mere revision; i.e. those I actually translated from scratch.
There are several out-of-box tools that you can use to check the differences between two or more documents. I typically use the built-in merge feature of Mendeley that gives you a confidence interval when merging two documents. From that you can extrapolate if the documents are pretty similar or extremely different.