DAZZA501: They never know exactly how fast the wind is blowing though.
While that is most likely true (i.e. that they don't know the precise number), I have to believe that experienced caddies are able to determine fairly closely what the speed of the wind is. Before they go out on the course, they have a lot of information about what to expect.
And while on the course, they have a lot of data (leaf or branch movement, movement of the flag, as well as their own experience) available to them to help determine the approximate speed of the wind.
I would think the biggest variable that they can't know with any precision is when a gust of wind might occur and how strong it will be.