Hello,
Did you Search the Clients with the "Search client" Feature?
There is another Method.
You can run a Computer Status report and export the results of the report.
Open the report with Excel and remove all fields except for the Computer Names.
https://www-secure.symantec.com/connect/articles/how-export-virus-definition-and-client-information-excel-format
You would be left with the Computer Names.
Check the machines which have not seen in the List.
Check 1 machines which is not in the List and check if SEP is installed on the same.
Once, you knwo that the machine is not carrying SEP, then you can assume these machines have not been installed with SEP.
Incase, if you see that the machine is Installed with SEP, then check where are they reporting to ?
This will be much more easier.