This seemingly innocuous question bears fruit when we realize that there is a whole field of study called epistemology – the ways of knowing. We tend to think that logic (cause and effect, the scientific method, etc.) is the only recourse we have to “knowing” something, but since “knowing” is a psychological concept, even a religious one, we must ask ourselves what we mean by “the answer” – if your philosophy begins with a belief that the universe is a know-able, concrete, physical one, then scientific methods may be all that is available to you, but if you are a small child, knowing that there is an Easter Bunny, a Santa Claus, a monster under your bed, or a good fairy, you are using a different set of “knowing tools,” somewhat the same as a religious believer in angels, etc.
So the answer to all things (questions?) is “what you believe to be true is true, by definition.”