As far as I know, that's usually how it goes, but you can find what's correct by knowing what it isn't. The process of elimination can give you the correct answer by…well, eliminating the wrong 'answers'.
I couldn't find a better word :(
What I meant was by defining what's incorrect you can get to a more accurate conclusion of what is correct based off of logical conclusions.
But something is incorrect because it is anything other than what is correct (i.e., incorrect != correct). But if correct is unknown, then incorrect must be unknown as well because incorrectness depends upon correctness. Criteria for what is correct is necessary to exclude anything and thus label it incorrect.
Also, where does truth factor into any of this? o_0