There is a philosophical discipline devoted to establishing Truth: epistemology. "Criteria of Truth" describe how people can, and do, make judgements on the truth of propositions, in the face of information which is incomplete, confusing, and out-of-date. Correspondence, coherence, and pragmatic theories are the strongest, though other fallbacks, including authority, tradition or custom, and majority opinion are used, despite the fact that they are unreliable.
The ultimate keeper of truth isn't quite the right question, but in general, those who are trusted with distinguishing truths are those who have shown the capacity, ability, and integrity to serve in that role. That is: who judges truth? Someone who is very good at doing just that.
Authorities aren't absolute, they're not perfect, and they're not immune to bias or corruption. But, over time, they should show themselves to be sincere in their efforts to uncover the truth, inclusive of admitting their errors and changing their judgements in the face of new evidence.
The problem is a complex one, but not a greenfield.