We use BrainTree a system powered by Paypal to verify your card.
Braintree describes the process as this,
"The gateway verifies credit cards by running either a $0 or $1 authorization and then automatically voiding it. For most processors, Visa and Mastercard are initially tried with $0 authorizations. Because some card types (like Amex and Discover) don’t consistently support $0 authorizations, they are only tried with $1 requests."