When switching on the power, the LEDs on the front of your Relay - 8 or on the PCB of the Relay - 4 will indicate the UNIT ID (between 1 and 4/8).
If you need to know the ID when it is higher than 4/8, then use a LAN or serial command to read it.
The command is: NEUNIT=1,UNITID=?<CR>