Not sure if there will be a way.  You need some way to test if a circuit is made on the ground shield and one of the wires to figure out the ground.  If you are really desperate and have a video game pad you could rip it apart as the contacts under the buttons are usually just two metal plates.  Touch one of the wires to one side to the button and the outside (ground) of the power plug to the other side to see if it completes the circuit and presses the button.  But that's the only thing I can think of, otherwise I'd go out and pick up a multimeter.