Answer by Mykola Veryha for Finding nodes that have not been indexed

We have a tracker to mark nodes indexed in the Drupal database in 'search_api_item' table.

\Drupal\search_api\Plugin\search_api\tracker\Basic

You can investigate the class. It will help to understand how to check if nodes were indexed.For example, after updating a node this tracker will be called from this method

\Drupal\search_api\Entity\Index::trackItemsInsertedOrUpdated

Be care with 'indexing_order':'fifo' => $this->t('Index items in the same order in which they were saved'),

It can make problems because we have this code in the tracker class:

    // Update the status of unindexed items only if the item order is LIFO.    // (Otherwise, an item that's regularly being updated might never get    // indexed.)    if ($this->configuration['indexing_order'] === 'fifo') {      $update->condition('status', self::STATUS_INDEXED);    }