How to use a multimeter to find positive or negative?

I'm not completely electrically illiterate but I have never gotten the hang of using a multimeter. Last year a halloween prop was broken. The head (which contains two dc servos; one for the eyes and one for the mouth) became disconnected from the body. The problem is that the wires disconnected from their weak point which is where they had been joined to the wires in the body which are color coded differently (this is a purchased prop, not one I made, and getting info from the manufacturer has been next to useless). The two pair of wires in the head are obvious (two complete sets each containing a black lead) but where they connect its just four random seperate colors: purple, red, green and white. I imagine there is some way for me to figure out with a multimeter which of the two leads are positive and which are negative, as well as which positive goes to which negative. Any help would be apreciated.