An auditor or private investigator, but granted, there's a lot of room for tax evasion - there's a reason a lot of businesses and contractors will gladly take cash, tax evasion being one, most digital transaction costing them money being another. It's fairly common in my country (NL) to see signs saying "please pay in cash" and "€0.25 surcharge if paying with debit card for amounts below €20"

Around where do you live? Anecdotally, I don't see much of any of that in Eindhoven or the surrounding villages.

Correlating with water and electricity bills could raise suspicion, but yeah, it's probably harder.

Maybe? Pre-digital machines would keep no tamper-proof record of usage. And a good variety of machines in your laundromat (with varying appetites for water and electricity) could discourage all but the most fanatical of auditors.