First, he rashly tried simply asking "Are you a Knight?" or "Are you a Liar?", but he soon realized that this would never work. Also, there were some natives who didn't even know what they were!
Then, being logic-oriented, he started with complex combinations like "What would you answer if I asked you...?" but most persons lost interest and didn't even try to work out an answer.
Then, he changed to a simpler "Is this shirt red?" pointing at his own clothes, but that failed when he met a daltonic person.
A simple "Are you talking to me?" also failed; most thought the answer was "No" since they had been no conversation; the logician just popped up and asked his question.
Arithmetic was no good either: most natives had no use for it, and couldn't even calculate 1+1. (And they also couldn't read, by the way.)
At last, he came upon a question that could never be answered "YES" by a Knight, or "NO" by a Liar.
What simple question (up to three words is "simple"; four or more starts getting complex) could he ask?
Can you also think of some other seemingly good questions, and how they could fail?