Hello,
There are 2 things -
1) Finding Clients without SEP client installed.
In this case, check the Articles below:
What does it mean to set a client as an Unmanaged Detector?
http://www.symantec.com/docs/TECH105722
Find Unmanaged Clients on a remote network location using the Unmanaged Detector
http://www.symantec.com/docs/TECH96234
Setting notifications when using the "Unmanaged Detector" feature in the SEPM
http://www.symantec.com/docs/TECH104897
2) Finding Clients with SEP clients and not reporting to the SEPM.
In this case, check the Articles below:
How to create a SEPM Custom Scheduled Report for Offline SEP Clients.
http://www.symantec.com/docs/TECH175948
Finding offline computers
http://www.symantec.com/docs/HOWTO81330
Hope that helps!!