To tell someone where the bear shit in the buckwheat means to explain something to them that they are very likely to know already, especially because it is self-evident.
«I'm not that stupid that I actually need somebody to tell me where the bear shit in the buckwheat: I can see for myself who the bad guy in the film is. Ta!»
Sarcastic response to any question that has an incredibly obvious answer. The humor lies in that you're mocking the person who askedthe question by jokingly asking a question that has an obvious answer. See "Is the Pope Catholic".