MasterCard makes its 270-page "Merchant Rules" available in PDF for public download at this page. Section 9.12.3 on page 2-22 reads in full "A merchant must not require, or post signs indicating that it requires, a minimum or maximum transaction amount to accept a valid MasterCard card."

They also have this page to report merchants who violate it. The first check box for describing the type of violation is "In order to make a MasterCard purchase, the merchant/retailer required a minimum or maximum amount."

I haven't checked other credit card associations or issuers, but I suspect others have similar information available.