If you have no wiring instructions and no idea of how a humbucker is connected inside, you have two options for finding out: you can either try to take the pickup apart - which I would advise against as the pickup can easily be damaged in the process -, or you can use an ohmmeter to measure resistance and then draw logical conclusions from it. To do this, switch a multimeter to the ohms-section, set the function switch to 20K ohms and bring the probes in contact with any two wires. If they are not connected, they belong to different coils. Continue holding one of these wires to one of the probes and test the others systematically through until a resistance value is shown, which means that you have found the two leads of one coil. Note their colors before you proceed in the same manner with the other leads. When you have found and noted the colors of the leads of the second coil, there will be one lead left which should be connected with the metal bottom plate. Quite often this wire is identical with the braided shield wire of the pickup cable and therefore instantly recognizable.
Excerpt from my book Building Electric Guitars