By doing no more than observing Kawaki's nape & arms, he has concluded that the boy's body was heavily modified.
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He even notices that Kawaki's blood vessels and nervous system were modified. Even if Kawaki had gashes, he would not be able to see that microscopic shit unless literal electricity & sci-fi-type blue lights were glinting out of his blood and gashes.
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HE CAN EVEN DEDUCE THAT KAWAKI WAS A NATURAL BORN HUMAN. How did he distinguish between ninja tech opposed to genetic engineering? With Byakugan level insight, that's how.
Could it be that he has modified himself with an artificial Byakugan of sorts that doesn't require his veins to strain themselves? Could it be that Boruto's eye isn't from a Hiyuga member, but actually synthetic??
Juugo's abilities are derived from a natural source though. The various methods all leave individuals looking somewhat similar when transformed. Sasuke only had the curse mark, Juugo's came from his blood, Mitsuki's is due to being made in a tube, and Kawaki was modified like a cyborg, yet Glasses Smart Guy is saying everything he's said in absolutes all while being quite specific as to what were changed.
Looks like the Byakugan lost it's worth, where's redundant logic when you want to rustle.
Ps. It's just the usual sub standard writing at work, Kodachi doesn't know how to write reveals so he had to have Katatsuke spout evwrything out of his mouth already.
Could it be that he has modified himself with an artificial Byakugan of sorts that doesn't require his veins to strain themselves? Could it be that Boruto's eye isn't from a Hiyuga member, but actually synthetic??