You need to follow the news. Also carefully examine graphs of the changes in the value of bitcoin. Learn and begin to apply trading indicators, they will help you see when the prices would be overheated, that is showing overbought. This will mean that the price will likely start to fall.
Graphs are shit if you are talking about the Bitcoin price, since the indicators, or charts can not do anything in order to predict or just give you an "idea" of how is it going.
Just buy, wait for some time, and you will earn.