1) Run either of these commands to identify the KMS Host:
slmgr.vbs /dlv
or
slmgr.vbs /dli
2) Logon to the KMS Host server as identified and run this powershell command as administrator, to export a text file with all of the registered KMS clients. Search the list for your Host servers or server that you're looking to confirm is using KMS Licensing.
$(foreach ($entry in (Get-EventLog -Logname "Key Management Service")) {$entry.ReplacementStrings[3]}) | sort-object -Unique >> C:\Clients.txt
Sources:
http://cloud.kemta.net/2014/08/powershell-listing-activated-clients-on-kms-server/
https://social.technet.microsoft.com/Forums/windowsserver/en-US/9a1ec2c8-eb04-4db9-b904-f8b65880ff1b/determine-kms-host-on-network
