Evading taxes would result in an inspection, and the method of tax inspection in this era was very simplistic. At the end of each month, City Hall would spend a month organizing accountants to verify one-tenth of all the tax filings from the previous month, checking whether there had been fluctuations in the amount of taxes and discrepancies in the names of items. If there were significant fluctuations and problems with the items, a specialized City Hall official would pay a visit to conduct an audit, or they would check things like gas meters to calculate production costs.
Although it sounds complicated, in reality, only units like Detective Agencies that weren't convenient for accurate business verification needed to pay special attention. There certainly were some loopholes for tax evasion, but Shard had no such intentions, after all, he was running a legitimate business.