The discovery was reportedly made after analyzing Monero’s nonce distribution before and after the past few ASIC mitigation hard forks. The individual pointed out that the network’s total hash rate dropped considerably right after Monero’s developers introduced measures to prohibit the use of ASIC hardware.

According to MoneroCrusher, the theoretical nonce distribution is a random number between 0 and 232. However, for a certain number of blocks prior to April 2018, most nonce selections were not randomized. Since blocks were being repeatedly mined with the same nonce, it became increasingly obvious to the researcher that a vast majority of the network’s hash rate was in control of one or very few entities. MoneroCrusher concluded that the pattern
could only be demonstrated by ASIC hardware.

Reference: https://beincrypto.com/hashrate-analysis-reveals-asics-account-for-85-of-monero-mining/