Hi,
You can use risk reports to find out if any new virus infected in the network.
Go to the Reports tab, Select Report type as a 'Risk', then under Selected report choose 'New risk detected in the network'.
If you believe there is a new threat in the network & Symantec is not detecting it then please run Sep support tool & submit the suspicious files to the Symantec for further analysis.
Here is the location of the Symantec Endpoint Protection Support Tool:
http://www.symantec.com/business/support/index?page=content&id=TECH105414
Make sure all the machines have latest definitions.
Windows machines should have latest patches/Service pack.
Disable autorun.
Refer these articles as well.
Security Best Practice Recommendations
http://www.symantec.com/docs/TECH91705
Best practices for responding to active threats on a network
http://www.symantec.com/docs/TECH122466
Security Response recommendations for Symantec Endpoint Protection settings
http://www.symantec.com/docs/TECH122943