hmm, well the key to building any signal set is to define what moves that market....for the currencies futures you could monitor the ICE listed dollar index future and the Yen, Eur, and Pound CME futures. From there you could grab the historical data, compute realized volatilities, find the average volatility, then construct a signal to trade when the daily volatility is higher or lower than some average in any of the futures assuming there is some daily correlation (you should compute that too).