Use a multimeter, set it to voltage. Hold the black and red wire against the poles of the battery. If it for example says 1.5 on the multimeters screen, then the red side is positive. If there is a minus sign infront of the numbers, then red is connected to negative pole. The black wire should always be connected to COM on the multimeter.
A multimeter is also good in contignuity mode to check for ground points (since all ground should be common).