If your meter has a "continuity" feature, check continuity to frame. The "negative" phase (-) cable will have continuity, the "positive" (+) cable will not.
The resistance measure of ohms will also be identical, 0 ohms to frame with negative cable, infinite ohms to frame with positive cable.
Or, power on converter, measure voltage by one probe to cable, the other to frame. Do not short the positive cable to ground, you will then be replacing fuses.
Good luck with your battery replacement. FWIW, I use a protective spray from a spray can on my terminals. IIRC, it came from Napa Auto Parts.
