The only ones that would have enough data to monitor for block withholding is nicehash and they'd have to keep track of every miner's share count and block finds and report the data - this is NOT a big task. As this should be nicehash's responsibility, especially considering the fee they charge, I think they're not providing an adequate service. No one has held them accountable for it and have been blindly buying hashes from them. The miners providing the hashrate stand to gain by block withholding as they get paid regardless of whether blocks are found or not, yet it is in their favour to keep bitcoin difficulty down. Buyer beware.