Mike,
 
I am not aware of a simple criterion one can use to tell a freshwater snail
shell from a land snail shell (or a gill-bearing snail from a lung-bearing
one) other than by becoming familiar with the shell types of freshwater and
land snails in a given location. However, if a live (or preserved) snail is
available for inspection, pulmonate (lung-bearing) snails can be
distinguished from prosobranch (gill-bearing) snails in most cases by a few
external characteristics. Prosobranchs have an operculum (a hardened flat
structure more or less shaped like the shell aperture) on the dorsal surface
of their feet that closes the shell aperture after the animal withdraws into
the shell. Pulmonates don't have opercula. Prosobranchs have one pair of
tentacles with eyes at the bases of the tentacles. Most pulmonates have 4
pairs of tentacles with eyes at the tips of the longest pair. However, there
are exceptions.
 
I hope this helps.
 
A.