I would take a 12 volt battery and some jumper wires. I would run the battery Ground directly to the trailer's frame. Then connect the positive to each of the three trailer wires and see which light lights up. If the black wire is ground then it will create a short circuit and there will be a big arc when you try to connect the wire.
The brake and turn signals use the same circuit.
Usually the colors are;
White = Ground
Brown = Tail lights
Yellow = Left turn
Green = Right turn
If you are trying to wire a car/truck that has separate turn and brake lights to a trailer where they are one piece you will need a Converter. These use diodes to change the currents to the tail lights. They are sold at most parts stores.
