How to check whether \Drupal\Core\Archiver\Zip::extract() succeeded?
The Drupal Zip class is just a wrapper around the PHP ZipArchive class. 

As @tenken already mentioned the wrapper just returns itself:

  /**
   * {@inheritdoc}
   */
  public function extract($path, array $files = []) {
    if ($files) {
      $this-&gt;zip-&gt;extractTo($path, $files);
    }
    else {
      $this-&gt;zip-&gt;extractTo($path);
    }

    return $this;
  }


I guess that your best option is to retrieve the ZipArchive object itself via the getArchive() method of the Zip class. On the ZipArchive class call the extractTo() method and check its output (TRUE or FALSE).

I am currently working on a rewrite of the archiver classes to use PharData: https://www.drupal.org/node/2891871. The PharData class throws exceptions on failure. If this issue is done and the patch lands you are able to catch exceptions to check if the archive operations were successful.