I see some people use fibonacci extensions as a way of predicting what price is the possible top of a rally before it would make a correction and some are able to predict what price it is in bitcoin just recently. This is just one of the indicators though because there are many things to consider when it comes to speculating the coins by doing some fundamental and technical analysis. When a coin is making a rally, you should know when its time to sell when you already have a strategy before the buying it like having a trailing stop loss so that in case it will have a sudden crash then you already secure your profit.