You need to enable sylink log to check that or from registry
https://www-secure.symantec.com/connect/forums/how-verify-sep-are-updated-gup
How to check which SEP client is not using the GUP to download content
You can enable sylink logging as well:
How to confirm if SEP Clients are receiving LiveUpdate content from Group Update Providers (GUPs)
You can also check the log on the client. View Logs >> Client Management >> View Logs >> System Log
There will be an entry showing the client is downloading from the GUP