Summary: Use Windows PowerShell to check the logon server of your clients.
How can I use Windows PowerShell to verify if my users are trying to sign in to their computers with a Windows account
instead of using their domain credentials?
Check the value of the environmental LOGONSERVER variable:
$env:LOGONSERVER
If it reports \\MicrosoftAccount, they are using their Windows account to sign in instead of domain credentials.
You can check this by using the IF statement as shown here:
if($env:LOGONSERVER -match "MicrosoftAccount") {"Logged on with Windows account"}