Well, what is sane? Certainly there shouldn't be an authority on it, however, ultimately we do know that any course of action is compatible with any level of intelligence - it just depends on what your goal is. But if we understand the goal then we can say something about the measures we take.
For example, if my goal is to visit Sidney then maybe acquiring a pogostick for that purpose... might not be the best idea and a boat or a plane might be. If my goal however is to have some fun then maybe (depending on what i find entertaining) may be the right idea.
But then epistemology also comes into the picture, we dont have perfect knowledge, there are different approaches to epistemology and of course i dont know what kind of experiences you have which may or may not make your believes reasonable even if we assume the same approach to epistemology.
So, yeah, there is not a way one could easily come to the conclusion about someones sanity.