In that situation, there's a big difference between 120 volts and 12 volts. For 120v I don't fool around and follow all the above advice.
For 12v, if I don't have a multimeter handy, I take the wire I think is positive and touch it briefly to something grounded with a quick swipe. I usually miss a few times, because I want it to be a very quick swipe. Anything more than a quick swipe can blow fuses. If there's a spark it's 12v (+12v on negatively grounded boats). If there's nothing around that I know is ground, I'll run my own wire to something I know is ground. If you have a multimeter, use it instead of this technique.
Then again, I have an Electrical Engineering background and am very familiar with these things. If you are unsure or not familiar with these things, get the multimeter. If you aren't absolutely sure what voltage you have, then no fooling around, use a multimeter. Actually, my lawyer just called me while I was typing this and told me to say to add one more sentence. Just get the multimeter.
Also, with 12v you still want to make sure you don't overload the circuit, heating up wires and making for a really bad day. Make sure you are connecting to a fuse. (If you pull all your fuses and flip all your breakers and the light stays on, address that problem first.) Look at the wattages of what you are connecting in total, and stay well below the current rating of the fuse/curcuit breaker. Current(Amps)=Watts/13.5v should work since 12v is really higher, especially when charging.