Goto console and login with admin id
Console -> Admin tab -> Licenses tab
There right handside license serial number will be reflect.
For client go to path - C:\ProgramData\Symantec\Symantec Endpoint Protection\12.1.3001.165.105\Data\Cached Installs (SEPxx.slf) .slf file will be there, edit the file in notepad and find the serial key.
serail key start from "M"
Check the article
https://www-secure.symantec.com/connect/articles/sep-121-and-license-concept