Look at the water's surface, see what's going on in front of the approaching cloud. The bigger, darker ones usually have built up to the point where they may have a downdraft coming out of the base. Now, is that cloud moving towards you (the outflow will be stronger if it is) or away from you? If it's early in the day, chances are that cloud is being formed buy rising hot air, so it's not going to help you too much, unless it's a huge building thunderstorm, in which case it will be sucking in a lot of air, right up until it "explodes" in a big downburst.

Sturart Walker has an written entire book called "The Sailors Wind". Check it out.