From BrickStorSP (gui client)
Click on the Domain name of your BrickStor on the left hand panel. Go to System and under the "Hardware" heading look for "Serial Number."
From the Command Line
Run the following command an look for system "SystemSerial"
# bsradm smb
Physically on the BrickStor Head
There is a pull out tab printed with the system serial. It is generally on the front right of the system, beneath the USB ports.