As you can see, cavities were on the decline prior to fluoridation. Even after fluoridation, the rate of decline is approximately the same. If fluoride was effective, there should have been a very noticable drop in the cavity rate.

50 year decline in tooth decay of 5-year-olds. Source. -- compiled from Health Department records of 5-year-olds' tooth decay 1930-1990, fluoridation and fluoride toothpaste.

From "WHY I CHANGED MY MIND ABOUT WATER FLUORIDATION"
John Colquhoun © 1997 University of Chicago Press.