There are a couple easy ways to check if memcache is actively being used on your site.

1. Enable the memcache_admin module provided with the memcache module.
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   Performance and     Memcache (memcache)                                   Module  Enabled   8.x-2.0
   Performance and     Memcache Admin (memcache_admin)                   	   Module  Enabled   8.x-2.0

   Once logged in to your site with an admin user account you should see the memcache statistics at the bottom of each page.
2. Log in to your NewRelic account . In the APM overview section if memcache is being used you will see Memcached graph as shown below:
   
3. There are more ways to check if memcache is being used by your site. Should the above not fit your needs please file an Acquia Support ticket.