I imagine there may be differences due to the effects of a stylus's friction when applied directly to the page, compared to the frictionless depression caused by printing through from one page to another. Perhaps the fibres are dragged in a particular way.

Also, did the writing implements of the time require pressure? With a quill, a brush or an ink pen with a nib you wouldn't be able to use that much pressure. So perhaps that's another way they could figure out that this isn't print-through. But it's not explained in the article, as you say.