In fact, they usually do not predict exchange rate movements very well. You can only rarely profit from exchange rate speculation To profit from exchange rate speculation you would have to be smarter than the markets. That requires years of experience and a bit of luck as well. If markets make the best use of information, and you do not have superior information, you cannot profit from exchange rate speculation. However, there is one scenario in which you can make a profit with great probability.
Many governments maintain limited-flexible exchange rate regimes which are prone to realignments.
You can profit from such realignments if you can correctly predict the timing of these events. Essentially, you are exploiting the fact that a government is working against market forces. Cambridge University Press, New York, 1989. Exchange Rates and International Finance . Exchange Rate Forecasting .