you could add a check constraint on the table. Something like this:
alter table payments
add constraint CK_PaymentMethod check
(
(PaymentMethod = 'Credit Card' and CC_Num is not null and CC_Name is not null and Check_Num is null)
OR
(PaymentMethod = 'Check' and CC_Num is null and CC_Name is null and Check_Num is not null)
)
Be One with the Optimizer
TG