You cannot compute a block's difficulty from its hash. The hash is a Keccak hash.
A different hash using Cryptonight is used for PoW purposes, but this isn't the block hash. It is often called the PoW hash, or just PoW. It uses a similar system to Bitcoin where the hash as an integer must be below some treshold based on difficulty. This hash is run on the "hashing blob" of the block, which is basically the header plus the merkle root of the transactions. So in order to compute the difficulty, you have to have the block hashing blob itself.
This PoW hash is visible to casual users when a reorg happens. You see messages such as:
2017-08-25 04:28:11.472 [P2P6] INFO global src/cryptonote_core/blockchain.cpp:1411 ###### REORGANIZE on height: 1384066 of 1384066 with cum_difficulty 3031277306080642
alternative blockchain size: 2 with cum_difficulty 3031295878143490
2017-08-25 04:28:11.846 [P2P6] INFO global src/cryptonote_core/blockchain.cpp:1422 ----- BLOCK ADDED AS ALTERNATIVE ON HEIGHT 1384066
Note how the PoW above is a small number (all the zeroes at the end).