If the authentication is broken, I can tell it's getting stuck negotiating the connection, because it displays a 'NEGO' status in the 'Authn' column.
In Windows Outlook, I've been able to check if a client is properly authenticated by checking the 'Authn' authentication column in Outlook's 'Connection Status' window (See Microsoft's documentation here). This has caused a lot of issues for users, with previously configured mail clients not properly negotiating connections with the mail servers. At the University where I work, we've recently made the switch to Modern Authentication.
