How to get commerce billing info fields
If I understood you correctly, you want to get a list of the form fields that a user is presented. The isn't really a clean way of doing that, since Drupal forms can be complex. The best solution however would be to generate the form and traverse it. To generate the form, you could do something like this:

$form_state = array();
$fields = array();
$profile = commerce_customer_profile_new('billing');
field_attach_form('commerce_customer_profile', $profile, $fields, $form_state);
// $fields is now the customer profile form.