How to know the "name" of an input field of a commerce checkout form? I need to find the name of an input field of a commerce checkout form to use in:

form_set_error('NAME-HERE', t('Field cannot be left blank'));


The html of the field is:

&lt;div class="field-type-text field-name-field-cif-nif-id field-widget-text-textfield form-wrapper" id="edit-customer-profile-billing-field-cif-nif-id"&gt;
    &lt;div id="customer-profile-billing-field-cif-nif-id-add-more-wrapper"&gt;
        &lt;div class="form-item form-type-textfield form-item-customer-profile-billing-field-cif-nif-id-und-0-value"&gt;
             &lt;label for="edit-customer-profile-billing-field-cif-nif-id-und-0-value"&gt;CIF/NIF/ID &lt;span class="form-required"&gt;*&lt;/span&gt;&lt;/label&gt;
             &lt;input class="text-full form-text" type="text" id="edit-customer-profile-billing-field-cif-nif-id-und-0-value" name="customer_profile_billing[field_cif_nif_id][und][0][value]" value="" size="20" maxlength="20"&gt;
        &lt;/div&gt;
    &lt;/div&gt;
&lt;/div&gt;


I tried with all of these, but it didn't work.

customer_profile_billing[field_cif_nif_id][und][0][value]

customer_profile_billing_field_cif_nif_id_und_0_value

customer_profile_billing-field_cif_nif_id-und-0-value

customer-profile-billing-field-cif-nif-id-und-0-value

customer_profile_billing_field_cif_nif_id


If I use 

dpm($form_state['values']);


I get this:
Give a try to this: 


  form_set_error('customer_profile_billing][field_cif_nif_id', t('This field failed validation'));