/**
 * 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['#validate'][] = 'utilities_custom_validate_required_field';
      }
}

function utilities_custom_validate_required_field($form, &$form_state) {
  // Check if the field you want to validate exists in the form.
  // replace 'field_dispongo_de_' with the name of field you want to validate.
    if ((isset($form_state['values']['field_cif_nif_id'])) && (empty($form_state['values']['field_cif_nif_id']))) {
    form_set_error('field_cif_nif_id', 'Field cannot be left blank');
    }
}