In hook, determine if processing a New customer or an Existing customer

In a hook I wrote, I am validating a custom field against a database table to ensure the value in the custom field is unique (not already used in our database)...

add_hook("ShoppingCartValidateCheckout",1,"hook_subdomain_validate");

As you can see, I'm doing this at the hook point 'ShoppingCartValidateCheckout'

It works great for new customers, but when there is an existing customer, I don't want to do this check as it is not necessary, and by definition it will fail, and is causing problems of course when this occurs. I need to by pass this check for existing customers.

Problem:

In my hook code I need to know how to determine if I'm dealing with a New or Existing customer so I can do nothing and return a positive response code if its and existing customer, effectively bypassing the validation logic in my hook.

Questions:

1 - How can I determine in my code if I am processing an order for a New customer or an Existing customer?

Hey all, sorry, I figured it out... Doing a var_dump of $vars in the hook, I see a variable $vars["custtype"]

Not sure how I missed it the first time, but its there, right in front of my eyes...