checkOrderHasBeenPaid.php

// inside an event subscriber of `OrderEvents::ORDER_UPDATE` (edited)

if ($order->getState()->value == 'completed' && $order_total = $order->getTotalPrice()) {
  $original_order = $order->original;
  if ($original_order && $order->getState()->value == 'draft') {
    /** @var \Drupal\commerce_payment\PaymentStorageInterface $payment_storage */
    $payment_storage = \Drupal::entityTypeManager()->getStorage('commerce_payment');
    $payments = $payment_storage->loadMultipleForOrder($order);
    if ($payments) {
      $payments_total= new Price(0, $order_total->getCurrencyCode());
      foreach ($payments as $payment) {
        $payments_total = $payments_total->add($payment->getAmount());
      }
      if ($order_total->equals($payments_total)) {
        // do stuff

      }
    }
  }
}