You can't but how can you prove the blockchain hasn't been tampered with?
Proof of Work, chain of blocks.
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.
Make it public and put it in version control, every time a vote is counted you publicise a new revision, everybody with an interest can download regularly and check for tampering?
This is a more complicated and less scalable way of creating a blockchain.
Or, you know, use a hash of the previous row as a column value of the current row. That way, you can’t tamper with any data already stored in the database.