CardCheck verifies credit card numbers to ensure they are genuine. CardCheck supports: Visa, MasterCard, American Express, Diners Club, and Discover cards. Cards can be checked individually or in batches. It performs three functions: 1. It checks the number of digits is correct for that card type; 2. It checks the leading digit(s) is correct for that card type; 3. It checks the validity of the number structure against the internal credit card checksum.