Most times you don't know its when it happens that's when a lot of people will start looking for reasons why there is either a fall or rise in price. However, to know is to be conversant with news and be ready to act fast on it but just know its a risk because its still a prediction that is not a respecter of your opinion as well. Just know what you are entering into.
We can't predict the price of bitcoin, when it will rise,when it will fall. Maybe, the least thing we can do is to monitor the flow of its market. Be observant and listen/watch news about bitcoin. Bitcoin's market supply and demand is the basis when it will increase and when it will decreases.