What makes a Council's conclusions authoritative?

I was just reflecting on an answer to a question I recently asked, and began to wonder... **How do we know the conclusions of a given Council were true?** Are all findings by Christian Councils authoritative? If not, what is the criteria by which we can judge the findings of a Council? Is it decided democratically amongst those claiming to be Christians? If there is indeed a "standard" by which Council findings are evaluated, why would we consider the Council findings as "authoritative", as opposed to simply considering the "standard" to be authoritative. *(The answer may not be as straightforward as it seems.)*