OK, sure, but you can look at bid/offer spreads across different market makers to get some sense of how liquid and efficient the overall market is. For GBP/USD,

OK, sure, but you can look at bid/offer spreads across different market makers to get some sense of how liquid and efficient the overall market is. For GBP/USD, spreads are measured in small numbers of tenths of a penny. Forex is essentially the largest, most liquid market there is.