You want to know what alerting Symantec Endpoint Security (SES) provides if the AD Gateway configured for Threat Defense for Active Directory (TDAD) were to fail.
Symantec Endpoint Security (SES) with TDAD enabled.
If the AD Gateway fails, there is no built-in alerting.
If you suspect that your AD Gateway has failed, check the client details in the console, specifically the Client Connection Status. If the gateway has failed, that status is "Offline".