You cannot, but with the two forks you can see that someone tried to tamper with the signature of a given video as there will be conflicting signatures. That alone can make tampering unattractive for an adversary.

How so? What if all I want to do is cast doubt on a legitimate video? The point of the video manipulation is to manipulate the human response and casting doubt is just as effective. Moreso since the average person is not familiar with the technology & thus defaults to "eh - it's all fake" because there's no way they can distinguish the likely real from likely fake from "too hard to tell".