* NPR story on Southpaw Snails 'Dodge' Right-Handed Crabs
You can get into counting snails with your local stream by joining a Stream Team.
You can tell the difference by holding the shell so that its tip is upward and the opening toward you. If the opening is to the right of the axis of the shell, the snail is termed dextral -- that is it is right-handed. (snail on the right) This type of snails are very sensitive to pollution. They need plenty of oxygen to survive. If the opening is to the left of the axis of the shell, the snail is termed sinistral -- that is it is left-handed. (Snail on left)
Right-handed snails are also known as Prosobranch, or gill-breathing snails. They possess a chalky plate called an operculum on top of their foot, which they use like a door to close the shell opening. The length or width of the shell ranges between 2 to 70 mm. The great majority of shells are black, brown, tan and gray in color. Most gastropods are vegetarians, eating algae, and dead and decaying plants. Most often snails can be found in waters less than 3 meters deep in more calmer parts of the stream. (from Ohio.edu)