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 checksum