There are some simple, uncontroversial metrics out there: logical consistency and consistency with real world data.
Ethical beliefs are based on reasons and facts. Ones that do not are baseless, and not worth believing. The remaining ones can be judged based on our two metrics. Is the logic consistent? If no, then the belief is false. Are the facts they claim consistent with reality? If no, then the belief is false.
For example, a member of Al-Quaeda might say that women should not be educated because they are all too stupid to learn anything. This is demonstrably false, since we have successfully taught very many women in the West. Accordingly, their belief - at least on this basis - is false.
These metrics are universal, despite not knowing the answer to life, the universe, and everything, and do successfully distinguish between some true and false ethical systems. That doesn't mean finding the one, true moral system is easy, but it DOES mean that not everything is relative.