Public water fluoridation has well-understood health benefits (a reduction in cavities that cannot be achieved through brushing alone). It also has well-understood drawbacks (tooth mottling in high concentrations, toxicity in high concentrations).
Someone participating in this discussion would either know this from general knowledge, or could look it up.
It's trivial to find sources for the effectiveness of fluoride in preventing tooth decay (Google -> Wikipedia -> a citation leads you here: http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5014a1.htm). There are also studies on water fluoridation's impacts, though it's harder to test that, by nature.
Someone participating in this discussion would either know this from general knowledge, or could look it up.
People aren't that ignorant.