How can we know that everything is predetermined?
The simple answer is that everyone will have their own standard of proof. There is no proof that can be given in one shot that will prove to everyone that everything is predetermined.
As part of this teaching, the notion of predetermination is not there so much to prove that the story has already happened, but more so to challenge the ingrained and inaccurate counter belief that the individual is in control of what's happening in life.
The realisation that the individual is not in control becomes more obvious and beyond doubt as the mechanics of life are observed earnestly. It can become seen that seeking unfolds magically - an intricate orchestra of coinciding events, contemplations, realisations and insights which are clearly outside of the individuals doing and also not possibly happening by accident - seeing this time and time again, becomes our own proof.