Through the GUI:
Look at the top right of the GUI window: the serial number, software and hardware revisions are listed there.
Looking at the Master:
The hardware rev level and unit's serial number are marked on the case, in permanent marker.
They are also marked on the PCB: remove the AC connector, look in the opening between the header and the case: the serial number is written in red marker on the PCB.
The first letter is the hardware revision level.
S/N and revs