There are any number of indicators for wind on the water, most of which should have been covered in your float rating.
As fantome said, it's something you get an "eye" for with practice. If you're having trouble judging the direction then try to use as many things available to you as possible, smoke, boats, flags, birds, foam streaks from waves etc.
Learning a bit about sailing won't hurt you either, not only will it help you understand a bit about reading the water, but it will also give you and understanding as to how the sails of a boat are positioned in certain wind conditions, another good indicator of wind direction when there are sailing boats around.
Also remember that some local winds may change direction quite quickly when they follow coastal contours or mix in with a sea/land breezes and may not always be the same as the predominant wind you've observed.