The Conundrum of the Secret City

People may have read different similar versions of this riddle about an explorer on his way to the secret city of, say, Njug. As he struggled through the jungles inhabited by two intermingled tribes - one of whom always lied, while the other always told the truth - he came to a fork in the road. There a native squatted. The explorer was minded to ask his advice but, as the locals all dressed identically, could not tell to which tribe he belonged. In a necessary refinement of the riddle, the tribes shared a further custom: they ate anyone who asked more than one question. How could the explorer formulate one enquiry so as to elicit an useful answer - which pointed him to the correct road to Njug?

As a matter of fact, I just copied the riddle directly from Felipe Fernandez-Armesto's "Truth: A History and a Guide for the Perplexed" - a thought-provoking book on the history of the concept of truth, and the quest for techniques for telling truth from falsehood. I have actually bought it almost ten years ago and been reading and pondering over different passages on and off. In our contemporary age of corporate cliche and platitude, political rhetoric, and financial and economic crisis, as particularly exemplified through the news media, a mental trip to the arena of truth is always refreshing and enlightening.

One possible answer the explorer might have tried to elicit from a liar was, "IF you were to ask me which is the way to Njug, I should say it was to your left." The answer would be false (as he was a liar), but it would point the explorer in the right direction, for the truth-teller's answer would be the same.