From the technical side of it.
For the older paper, most of the wood pulp based stuff that has visible darker fibers, Those are absolutely a fingerprint. And one that can easily be read without technolgy.
On some papers even without darker fibers, like T206, it's harder to see but magnified the fibers are visible. And those as well create a unique pattern.
Modern stuff typically has been bleaced, and has much finer fibers, I'm not sure it's possible to use the paper on something like 89 Upper Deck.
Print defects are not as conclusive. The entire goal of printing is to produce multiple identical items, so many defects are common and consistent. Just looking at the WWG Dimaggio, after about 10 different ones I was able to spot tiny differences that were consistent. There were probably at least two different plates, or like other sets there were two different places on a larger sheet that were that card.
Some print defects are more transient, and can be more likely as identifiers.
On a few sets there are marks in individual positions that can be used to determine authenticity.
For example, the remnants of transfer alignment marks on T206 Magie, correlate to consistent print defects on the reverse, and neither match a Magee.
My wife is a computer developer, and has assured me that having a computer compare two pictures to see if they match or not is old established technology.
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