I don't think traders actually know? They just assume, predict, foresee it. It's not a guarantee but they have tons of information to back up their speculations together with their various experiences with the market, which pretty much reveals their personal judgment on how the market would move. Emphasis on the person though, since no judgment could actually be judged if it's true or not before the said event has come to pass. A prediction is just that, a prediction, it has no stamp of guarantee that it would actually happen.