How can a smart contract deployed on Mainnet know if it is running on PoW or (in the future) PoS?

Question

I'd like my smart contract to know if it's running on PoW or (in the future) PoS in Mainnet. Does checking for block.difficulty solve the problem?

If so, what will be the value of block.difficulty after Mainnet moves to PoS?

Answer 1

Difficulty will be 0 (zero), see https://eips.ethereum.org/EIPS/eip-3675#block-structure.