// Check previous GPIOs with a Logic Analyzer
void setup() {
  Serial.begin(115200);  // sets default RX and TX
  Serial.println("This will go out in the default TX pin");
  Serial.flush();
  
  Serial.setPins(-1, 2);  // Now TX shall be GPIO2 and default TX GPIO can't send any data
  Serial.println("This line can't be seen in the default TX GPIO!");
  Serial.flush();
}

void loop(){
}