Experience is the best answer when it comes to this. People have gotten answers from lucid dreams they couldn't have possibly known on their own. Like the exact year a couple would break up, or the first five words that would come out of a person's mouth the next time they saw them.

I believe it can work for everybody, but we're naturally skeptical so it's best to verify for ourselves first. Try testing it with simple questions like the birthday of someone you don't really know, a middle name, an outfit you wore in a picture you can't consciously remember, things like that. And if turns out to be true, then no one can promise it will always be right, but you'll know it can be.