He knows which way the river flows, observes the current, and deduces west / east.
The doctor is his mother (Tin Cup)
A traveller walks down a road and comes to a fork. Down one fork, is Truthville, where all citizens cannot help but tell the truth. Down the other fork is Falseville, where all citizens must lie, always. There is a man at the intersection who is a citizen of one of the two towns.
The traveller asks the man one simple yes or no question, and by the mans answer, the traveller knows which fork goes to each town.
What does he ask?