How to test if memcache is working? After successfully enabling it, how do you know it's working? And how do you know how much better performance is increased?

In homepage of https://www.drupal.org/project/memcache it says:


A module that provides a comprehensive administrative overview of
Drupal's interaction with Memcached and stats.
A set of tests that can be run to test your memcache setup.


But there is nothing for Drupal 8.

If you read docu http://cgit.drupalcode.org/memcache/tree/README.txt?h=8.x-2.x there is no information how to test/confirm memcache is doing its job.

It shows in Status report it's enabled. Memcached version  3.0.0 But how do you know it's not just hanging around or idling?

Anyway, I read the 7.x docu and it had this:

## CACHE HEADER ##

Drupal core indicates whether or not a page was served out of the cache by
setting the 'X-Drupal-Cache' response header with a value of HIT or MISS. If
you'd like to confirm whether pages are actually being retreived from Memcache
and not another backend, you can enable the following option:

  $conf['memcache_pagecache_header'] = TRUE;


So I put in $settings['memcache_pagecache_header'] = TRUE;, but I don't see anything in my Network tab of Firefox's dev tools. All I see is X-Drupal-Cache UNCACHABLE with or without the setting.
You can enable the memcache_admin module and this will provide a stats page at /admin/reports/memcache, where you can see the number of sets and gets, hits and misses percentages and how much of the allocated memory is in use.

On the server, you can run this from the command line (assuming port 11211):

echo stats | nc 127.0.0.1 11211