How I can know if a buyer is a registered user?

How I can know if a buyer is a registered user?

Hello, I'm trying to make a payment is only available to registered users. How I can I know during the checkout process if the buyer is registered.
I have enabled the option to buy without registering.
thank you very much

Aw: How I can know if a buyer is a registered user?

I found how to do it

$user = &JFactory::getUser();
if ($user->id) {
print " USUARIO REGISTRADO " ;
}
else {
print " USUARIO SIN REGISTRAR" ;
}
?>