Warning message if shipping=A and payment=X

Hi guys!
I have two shipping method options (A,B) and I have two payment options (X,Y).
I want to be able to a warning message( $json['error']['warning'] = "blablabla") if the shipping method=A and the payment method=X.
I try to modify ...catalog/controller/checkout/payment.php but no result.
thanks for the help!