No single person, group or authority. It is an ongoing determination by many people from a host of backgrounds, biases and intentions who still all agree on what is reasonable and what is not. It is determined by merely asking oneself "do I have doubt that is logical, fair and as unbiased as possible?". Gather enough of these people - from as wide a field as possible - and you have reason.
> it is an ongoing determination by many people from a host of backgrounds, biases and intentions who still all agree on what is reasonable and what is not.
Dialectics. Immanent critique. The method of isolating the contradictions of a system so as to arrive at the best possible answer. No one said there is a quick formula for this, but that doesn't mean it shouldn't be attempted.