Testing wiring for positive or negative

I have a trailer that needed a new battery. I took all the wiring off. Came back after getting a new battery and forgot to take a picture of how it was wired. Besides red positive and black being negative how can I determine what the white wire is? Can a take a multimeter set it to Ohms and stick the trailer frame and each wire to see what’s common and determine if the wire is positive or negative?