Let me ask you this point blank: suppose you have a dream and you are sure your God is talking to you and He tells you to kill your own wife. Would you do it? See the dilema your delusion causes here? If you don't do it, it means you really don't believe in God. If you do do it, you are insane.
Good question, Jeff—and one I addressed here recently in the context of Mitt Romney’s Mormonism.
According to Christian teaching, God cannot and will not ask us to violate morality, because good and evil are absolutes, and he is all good; in the same way that he cannot and will not violate the rules of logic.
Therefore, if I had a dream in which “God” told me to kill my wife (assuming this was obviously immoral—not all killing is), then by that I would simply know that the dream was not from God.
In general, for Catholics, the rule is to be quite wary of such private revelations; to “test the spirits.” Because, of course, we believe in the existence of evil spirits as well as good ones, and evil spirits too can send dreams and visions.