If you are using a KMS server for activating servers and clients in your environment, you may have noticed that there’s really no obvious way to get a list of all the clients that have been activated by the KMS server.
One way to get that overview is by using VAMT (http://technet.microsoft.com/en-us/library/hh824953.aspx), but since that tool is based on pulling info from clients and not from the KMS server it is not suitable for everyone.
Thankfully, there’s PowerShell 
Getting a list of all activated KMS clients through PowerShell is actually a simple one-liner:
$(foreach ($entry in (Get-EventLog -Logname "Key Management Service")) {$entry.ReplacementStrings[3]}) | sort-object -Unique
What this does is look through the Key Management Service eventlog, grab only the client name and then remove all duplicates (since a client activates itself at regular intervals, there will be duplicates).
https://cloud.kemta.net/2014/08/powershell-listing-activated-clients-on-kms-server/