If you do not wish to purchase a meter you can get by with a 12volt test light. Put your headlights on and put one leg of your test light on one side of a connection and the other leg on the other. Even simpler is to ground one leg of the test light and go along the wiring until the 12 v is lost.
You can either buy a test light or solder two leads to an old tailight bulb. Also possible to leave it in a spare socket and extend the leads.