Verify that salt.txt was located and being used by Drupal? In my settings file in Drupal 8, I am specifying a location for hash-salt to a file outside of the docroot.

Example:

$settings['hash_salt'] = file_get_contents('/salt.txt');


I don't see anything in the status report or way to verify that Drupal was able to locate this file on the webserver and it is using it. I tried setting it to a broken path and/or file that doesn't exist, I even removed the line. However I don't see any noticeable change, so, how can you tell that this setting is being respected?
You can check this with drush by using Settings::get('hash_salt'):

drush ev "var_dump(\Drupal\Core\Site\Settings::get('hash_salt'))"