If there are two wires coming out of a light, then they both just go to the battery. If there is only one wire, then the light is grounded to the chassis and you'll need to connect it or the chassis to the negative terminal of the battery. Unless of course it's a dual intensity light, in which case there will be 2 hot wires and one ground wire or chassis ground.
Either way it would probably be safer just to ring out the wires with a meter. The only way to really test all the wiring though is when it's in its final state, so battery actually in the car and all the wiring grounds hooked up where they're suppose to go.