/**
 * Implements hook_form_alter().
 */
function utilities_form_alter(&$form, &$form_state, $form_id) {
    if ($form_id == 'commerce_checkout_form_checkout') {

        $form['customer_profile_billing']['field_cif_nif_id']['#states'] = array(
          // This #states rule says that the "field_cif_nif_id" fieldset should only
          // be shown if the "field_dispongo_de_" form element is set to "CIF/NIF/ID".
            'visible' => array(
                '#edit-customer-profile-billing-field-dispongo-de-und' => array('value' => t('CIF/NIF/ID')),
            ),'required' => array(
                '#edit-customer-profile-billing-field-dispongo-de-und' => array('value' => t('CIF/NIF/ID')),
            ),
        );
        $form['buttons']['continue']['#validate'][]  = 'utilities_custom_validate_required_field';
    }
}
/**
 * Callback handling the custom validation.
 */
function utilities_custom_validate_required_field($form, &$form_state) {
      if ((isset($form_state['values']['customer_profile_billing']['field_cif_nif_id'])) && empty($form_state['values']['customer_profile_billing']['field_cif_nif_id']['und'][0]['value'])
      ) {
             dpm($form_state['values']);
         form_set_error('customer_profile_billing[field_cif_nif_id][und][0][value', t('Field cannot be left blank'));
      }
}