%programfiles%\Symantec\Symantec Endpoint Protection Manager\data\outbox\agent\{Group ID}\Config.xml
Note: Several errors will be logged in the Windows Event Logs using method #2
Technical Information
You can confirm that a client has received the upgrade schedule by checking in the Config.xml file in the client program folder.
(example C:\ProgramData\Symantec\Symantec Endpoint Protection\CurrentVersion\Data\Config\config.xml).
This file will show the download window and version assigned for the upgrade.