You guys can't be serious. Get meter, adjust to 200V DC. plug in cords, usually red and black. If value is positive then red pin is positive, black is ground. If display shows negative ("-") then black pin is positive.
Then again, meters are expensive and the nearest may be hundreds of miles away. Bush fix.
Strip some wire, mains stuff will do, so you have only one single strand. Turn the power on, stick the strand in one of the plug connectors and connect it to earth. It it ignores this event, that's earth. If your hand gets warm and the wire disappears, that's +.
Worked for me.
