Easy. Just get a multimeter, and set it to 12V DC. Get the negative side (usually black) of the multimeter and firly press it against the chasis of the car, or any metal part you are certain is connected to the chasis (or even a black ground wire if you know of one). Then get the positive side (usually red) and touch each of the wires at the back.
I was wondering if anyone would know, or be able to point me in the right direction. I'm trying to figure out which wire coming out of the back of the radio is the ignition or 'remote on' so I can tap into. It's a 2007 Mazda 3 and it's the last wire I have to run. Thanks for any information in advance!