Pins 0 and 1 are serial pins which are also connected to usb, and you use them when you are programming your board or when you send anything to serial. It is unlikely that any shield uses it unless it has to and it although has a benefit of being able to listen to what the Arduino sends.
I think hitching a ride on pins 1 and 0 (rx/tx) would be the easiest. SO you can print or write to them the same way as you respond to yourself.
Just a warning that when you are sending data between two boards and Print numbers they come as symbols of the numbers, not the numbers themselves So
Serial.print(7) would arrive to the next arduino as number 55 or symbol '7'. If you are sending numbers between two arduinos you can use write
Serial.write(7); which does not change data, but your serial monitor on a computer wont read it right, or you can send them as symbols, but then the arduino listening may have to convert number symbol to the number. (sooner or later you will have to create yourself a simple function to convert symbols to numbers in your project...