Is there any to find the computers without sep installed in a lan network.
You can use the unamanged detector feature in the SEPM
http://www.symantec.com/business/support/index?page=content&id=TECH96234
Without SEP is called as unmanged client..
There is a presentation about Unamaged Detector in the Videos section:
https://www-secure.symantec.com/connect/videos/unmanaged-detector
The Easiet way would be to use Find unmaanged computer feature. You can also configure unmaanged detector, and then setup unmanaged detector notification/report.
https://www-secure.symantec.com/connect/idea/unmanaged-detector-functionality