How to find a Data Selector in a rule condition? Update 3: I just need to know how to find a Data Selector in a rule condition

In a Commerce Kickstart distribution:

To reproduce:

1) I added a billing field called: "VAT NUMBER" at: "admin/commerce/customer-profiles/types/billing/fields"?

2) I created a Sale Tax at: admin/commerce/config/taxes

3) For this new Sale Tax I clicked at "configure component" to edit the "rule component" for the tax. 

4) Now inside the rule: in Conditions I need to check if the field: "VAT NUMBER" is empty

Here is my problem: I don't know which is the Data Selector to get the field: "VAT NUMBER" and make the comparation.

I found this screencast which shows how to create a conditional tax based in the address field: http://www.drupalcommerce.org/videos/tutorials/drupal-commerce-condition...

I need to do the same that is shown in the video with the only difference that the condition for the video is based in the address field and in my case I need to base the condition to the value of "VAT NUMBER" 

UPDATE: This post is no the solution that I need, but it might help to put it in the right direction: http://www.drupalcommerce.org/faq/rules-field-data-selection

UPADE 2 I tried to use the condition "Entity has field" but I can't find which is the right Entity to select to get the condition to TRUE
Found the way:

To drill to the selector I have to create 2 "entity has field" conditions.

1) entity "site:current-cart-order" has field "commerce_costumer_billing"

2) entity "site:current-cart-order:commerce-customer-billing" has field "vat_number"

then I can select the data selector