objective morality

I've spent a decent amount of time over the last few years learning about religious argumentation through debates, lectures, and other formats and there's something on my mind at present which I can't wrap my head around: objective morality. Now, what's most commonly done (in my experience) is...

Suppose there are two people who both believe in objective morality. Person A says X is wrong. Person B says X is right. What method would you use to determine who was right or wrong?