If we know which printer the label is going to be sent to for print, then we can use ! U1 getvar "device.unique_id" to get the ID of the printer. Then, we can encode the ID in the ZPL as part of a label prior to sending the label to that printer.
If we don't know which printer the label to be sent to, then there is no way to retrieve the printer ID or add the ID after the ZPL has already been sent to the printer.
! U1 setvar "device.friendly_name" "PRINTER_NAME"
You can write any value to this variable and use it as an "identifier" if you use this command.
! U1 setvar "device.friendly_name" "" but do not specify a value, then the serial number of the device will be entered into it.
Or you can use getvar "device.unique_id" when printing a label