Generally, a letter from the customer indicating the specific percentage of revenues derived from each ineligible activity will suffice. (If there are multiple ineligible activities then the aggregate cannot exceed 50%.) A new letter should be obtained in connection with every annual review.
It's unlikely that the business' income tax return would break down the specific amounts of income attributable to lottery ticket sales, check cashing, etc. Your best shot at verifiable proof would be an income statement prepared by a CPA.
Remember, it's your bank that wants to recognize the customer as an exempt person. The customer probably doesn't care either way. At some point as you begin to pile on the documentation requirements, the customer is going to say, "Forget it, file the stupid forms."
My point is that I have no formal support for saying examiners are going to accept a letter from the customer except that I've never heard of their refusing to accept that level of documentation. If they want more, they have to be smart enough to know you may not be able to get it and the exemption (which they are supposed to encourage) may be lost.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.