Before handing payment information off to the processor, Full Slate checks that the card number and CVC have the right number of digits, it’s a card type you accept, it’s not expired, and address isn’t blank (if required). After that, it depends on your processor. Some processors will decline the charge if CVC and AVS fail, others will note the failure but accept the charge, and sometimes it’s configurable.
In general, the credit card information isn’t verified until a charge is submitted against it. Sometime people “authorize” a card for $1.00 just to make sure it’s valid, but Visa discourages this because it actually puts a temporary hold on the card. So Full Slate does not run a test transaction when a card is placed on file without payment.