There is no 'proof' it's part of the basic concept of PoW.

You would have to have enough hashing power to mine more then 6 blocks while the rest of the network only mined 6. So if nothing else changes you would need FAR MORE then current existing SHA256 hash power to ATTEMPT do it. And there is still the chance due to luck that it still would fail.

You can look at the amount of existing hash power and extrapolate from there.

-Dave