If you already use the serial port of an arduino to transmit data
to a program, you can't print debug statement and watch them in the monitor.
This uses a second arduino to forward debug data through digital pins using another
Arduino.

Upload this to an Arduino (debugger).

Connect the other Arduino (sender) and the debugger gnd together.

Assuming you're using pin 12 (RX) and 13 (TX) on the sender,
connect pin 12 (sender) to pin 3 (debugger) and 13 (sender) to 2 (debugger).
In other words, connect RX to the other TX and TX to the other RX.

To debug in the sender:

#include <SoftwareSerial.h>

SoftwareSerial SerialDebug(12, 13); // RX, TX

void setup() {
    SerialDebug.begin(38400);
    SerialDebug.println("Debugging");
}