In most case, you can not connect a wire to the PCB board on a finished product to get the operation log or debug.
This design allows you to obtain device information through a led, you only need to connect led to the printing device serial port.
This design tested with 9600 baud rate.
Connect a led to the serial port TX.
Build a amplifier board with a photo detector.
Connect the output to a usb-serial adapter or a oscilloscope,
and place the detector close to the debug led.