with a voltage meter set on dc, you will get voltage when you pair the + and - wires correctly (red/ red, black/ black) only one way, and nothing the other.
also, with a bulb or other 12 volt device, touch one wire to the bike's wire and the other to some steel on the bike body. If it lights, you have the hot red + wire. if nothing happens, you have a negative or dead wire.
make sure you have the ignition on if you are checking the tail lamp wires, and have the ignition on with the blinkers working if you want to connect to the blinker wires
otherwise you will never know what you have... especially if you forget to turn on the ignition!! (dont have to start the bike)