You have an advantage because you have other customers in the same business, they provide your baseline. Compare the make up of their receipts to those of this customer in terms of both amounts and percentages by the type of payments; e.g. checks, ACH, credit cards, etc.
Now, the hard part: determine whether their pricing and volume of business is comparable to that of their peers within your institution.
I'm aware of a SAR filing based on the absence of cash that played a pivotal role in a law enforcement investigation, but the SAR was only persuasive because it demonstrated that the customer's activity was wildly different than that of several peer businesses in the same immediate area.
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.