What would be the harm?

[The Grand Nudger]

I have one question for you:  How do you distinguish between the following cases:
1)  There's an objective moral truth, and not more than one person in an argument is getting it right.
2)  There's no objective moral truth, and two or more people have different ideas about how to think, act or feel about something.

In the case of 2, where two people are not referring to any fact, that get's discarded as opinion until such a time as it meets the bar.  Moral objectivity proceeds exactly as any assessment you've ever made between fact and opinion does.  It's a non novel system.  

There's also the case of 3 - there are moral facts but neither person is referring to one.  A difference of opinion.

You are probably familiar with my approach to truth: Truth-in-context™.  Take, for example, abortion.  GIVEN the sanctity of all human life, then abortion is wrong-- a human zygote is still human, and killing it represents harm to it.  GIVEN that the sanctity of life depends on experience, and that a zygote cannot experience, and GIVEN the sanctity of a woman's right to self-determination, then abortion is pretty much fine.

You're calculating relevant facts in order to reach a well informed conclusion - that's moral realism.  

My view of morality is that people will feel about things, will decide how they want things to be, and will go through a process of social negotiation, i.e. they'll cast their moral vote-- no objective truth required.  But how would you establish one or the other to be correct?  I don't think there IS a right answer to be found, even hypothetically, to questions like this.

There's certainly no requirement that any fact be referred to in any evaluation.  We make fact free and fact impoverished judgements all the time.  Those sorts of judgments simply aren't the sorts of judgements moral realists are going to sign off on.  The world is full of moral judgements...the vast majority of which do not qualify as realist judgements. Moral realism is a smaller set than moral subjectivity, because there can never be as many facts of a matter x as there are opinions on matter x.

The way to establish "which one is correct", in moral realism..is the same way that you establish "which one is correct" in any other factual assessment.  Which of the two has a greater command of the fullest set of relevant facts and can string that set in a valid inference to reach a sound conclusion. The right moral answer is no different from any other right answer in that regard.