The simple answer is – you can’t! Not reporting income is against the law.
You should consider only the income that the seller can show you. We all know, of course, especially in cash type businesses, that there is the possibility that the seller is not reporting all of his or her income for tax purposes. This “underground economy” has been well-documented and is in the billions of dollars. Many sellers will tell you about how much they are “skimming,” but you should ignore their statements, since they have no way of proving these amounts.
In determining whether a business is the right one for you, you should base the decision on the figures actually supplied to you by the seller.