How can I know if the customer that made the order is guest or registered customer?

as you said I am using the Mage_Sales_Model_Order object in order to get the information then:

if($order->getCustomerIsGuest()){
    // customer is guest
}else{
    $customerid = $order->getCustomer()->getId();
    //send the customer id to your external ERP
}

if you just want to know if the customer is guest or not. trigger getCustomerIsGuest() in Mage_Sales_Model_Order I believe that can get customer instance of order.