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You can't but how can you prove the blockchain hasn't been tampered with?

If you are asking, then you don't understand how the blockchain works.

It's literally the entire point of the chain.

Prove that your blockchain hasn't been compromised by a sophisticated 51% attack.

You can't. A 51% attack is literally "the network".

And I wouldn't call a 51% attack sophisticated, really. It's just having a bigger hammer than the other guy.

Proof of Work, chain of blocks.