You use the POH and any supplemental type certificates. Float planes in
particular are prone to change class from time to time as the floats come on
or off, so the only way to tell what class the aircraft is currently is
often to just go out and look at it. This makes the statistics kind of
fuzzy. The FAA might have some idea of how many airplanes it has issued an
STC for floats, but it has no idea without a survey of how many of those
planes are actually equipped with floats. Amphibians are a little easier,
but even here the FAA's position has been that if you are flying your
amphibian on land only, then its class for regulatory purposes is single
engine land. There are probably some Republic Seabees out there that are
flown regularly but have not seen water in years.