Knights always tell the truth,liars always lie,and knaves alternate between truths and lies.
Suppose you ask a native,"Are you a knight,a liar,or a knave?"
The native answers,and you are able to figure out what he is.
What is the native?
If the native answered knave, the native could either be a truth telling knave or a lying liah.
If the native answered knight, the native could either be a truth telling knight or a lying liah.
If the native answered liah, the native could only be a knave, because a knight would tell the truth and answer knight, and a liah would lye and not say he was a liah.
ANSWER: Therefore, the only possible way to tell who the native is, is if he answers liah, and would therefore be a knave.