How to find credit card you co sign for people?

I want to know how do you find credit card you co sign for people? My dad co sign some credit cards to a few relatives over 15 years ago and he forgot who it was and those he does remember, but forgot what company/bank it was too either. Is there anyway we can find out who he help co sign and to what company/bank? All we need to know is the card holder name and what company it is.