So either you generate some fake but valid credit card numbers (i.e with credit card number generator) and set some monitoring on them or even better, you gener

So either you generate some fake but valid credit card numbers (i.e with credit card number generator) and set some monitoring on them or even better, you generate a credit card number with something like revolut, and as soon you get some transaction on that number, you know your database was leaked.. not hard actually to setup something like that.