Given our best understanding of physics, to be able to *know* the future (the definition of knowing being "to believe without any doubt something that is true") of a person you have to know the complete proper present of that person, that is, the state of the entire universe that can affect that person's future (which is the whole universe, but increasingly further back in time the farther you get from that person). That's called being omniscient. Other than that, you can predict with increasing accuracy, but never with complete certainty.