There're multiple ways of finding it PowerShell and registry.
Registry is located Product Key -> Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\SoftwareProtectionPlatform ->BackupProductKeyDefault
Powershell - (Get-ItemProperty -Path "HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion\SoftwareProtectionPlatform" -Name BackupProductKeyDefault).BackupProductKeyDefault
Please look at the link below for more detail
Hope this helps
J