The VS1053 has 8 GPIO pins that can be read and written via the library. The player_gpiotest sketch demonstrates how to do this.
Run the player_gpiotest sketch
Connect the Arduino to your computer with a USB cable. Select File->Examples->Adafruit_VS1053->player_gpiotest
to load the example code.
If you have headphones, you will hear a beep at the start to indicate that the sketch is running. Then you should see the LEDs flashed in sequence.
If you open the Serial Monitor, you can see the values that are written to and read from each GPIO pin.