One sentence that throws the problem of evil out of the window.

[The Grand Nudger]

The important difference is though that there is no way for two people who disagree about a moral act to find out who is right.

I don't think that's true.  

Whereas two people who disagree about the causes of thunder for example, an objective fact, could in theory determine who if either are correct.

Objective moral theories approach morality the same way.  They ask the question, "what are we even talking about?" Then, when they think they have a good answer to that, they apply it to something out in the world, and see if any relevant facts of that matter exist, and what conclusions can be drawn from them.

If morality was objective as well then two people who disagree could in theory find out for sure who was correct. But they can't.

Sure we could...but it's important to note that, as our discussions with creationists, for example have shown us both...you can show a person what and how, and where to independently verify x....and they'll still tell you x is wrong, and can;t happen.  The same is true of an objective moral theory.  We could lay out our definition of terms, justify our definition of terms, then apply that definition of terms to what we can both objectively verify...and at the end of all that, be told that we still haven't demonstrated so much as the possibility of an objective morality.

Let's put it another way.

Subjective means "based on or influenced by personal feelings, tastes, or opinions""

Therefore something can only be subjective if a brain is involved to produce those personal feelings, tastes and opinions.

There is no other way to determine the morality of an act other than to use your brain. There are no measurements that you can possbly use to determine how right or wrong an action is.

Therefore morality can only be subjective.

Therefore all things produced by our brains are subjective.  That's not a meaningful subjectivity in any context, nor is it one that a moral objectivist has to deny.

Meanwhile, at least some of the things we take to be moral issues or moral facts of some matter x -can- be measured. I don't have to stop at the assertion..for example, that you feel pain. It certainly seems as though I might be able to devise an experiment to test that assertion. What do you think? Now, let me ask you a question. Is pain a relevant fact of the matter in your moral assessment of livestock processing?