<i>Then you can have as many &quot;customers&quot; as you like and nobody can really tell how many you really might have had</i><p>Note that, for this and related reasons, the IRS guides on examining car washes and laundromats (yes, these documents both exist and are publicly available) says to find their water bill from the business&#x27; books, look up the price of water in the area, Fermi estimate the amount of legitimate business happening, and compare with reported revenue.  If a wash costs $1 and you use 250k washes worth of water but report $60k in gross receipts you&#x27;re evading taxes; if you report $6 million in gross receipts you&#x27;re laundering more than underwear.<p>(A technique with similar impact is &quot;compare with a selection of similarly situated businesses in your files.&quot;)