In addition to Mike's answer, You can also say just as '_는/(으)ㄴ지 아닌지' And it would be more natural to switch the particles above. ;-) Ex. 나는 그가 나쁜 사람인지 아닌지 몰라요. I don't know whether he's a good guy or not.

Afaik, "whether or not" in Korean has close correlation with "know/don't know". For example, if somebody wants to assert another person a bad/good guy, s/he will say either, "I know that he's a bad guy" or "I don't know if/whether he's a good guy". Hence, "whether or not" is usually precedented by personal statement/assessment. The pattern is like this: V ~는/(으)ㄴ지 알다/모르다 Eg: 내가 그는 나쁜 사람인지 알아요 ( I know (whether/if/that) he's a bad guy) 내가 그는 나쁜 사람인지 몰라요 (I don't know whether/if/that he's a bad guy) 내가 그는 나쁜 사람인지 좋은 사람인지 몰라요 (I don't know whether he's a good guy or a bad guy)