Hi Mr. Trader,
Buy a cheap multimeter, you need one anyway. Set the meter to read DC. The meter has red and black probes. Probe the 2 black wires you have, it doesn't matter which lead of the meter you chose.
If the meter reads a positive voltage (12v), the black lead is probing the neg, (or ground) and the red the pos. + (or hot). If the meter reads negative (-12v) then the red it touching ground.