With that warning given, Potamopyrgus in the western USA is distinctive enough to be generally recognized, at least by researchers that are accustomed to working with 5 mm animals. The pictures on this page are linked to larger versions. See the photo gallery for more New Zealand mudsnail pictures.
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Operculum: Like all prosobranchs, but unlike the more common pulmonates, the New Zealand mudsnail has an operculum to block the shell aperture when the animal is withdrawn into its shell. This is easily seen on live snails (first 2 pictures above), but the operculum is lost from dead shells and it is normally withdrawn beyond view in shells that are directly preserved in alcohol or formalin (last 2 pictures above).
Color: The shell is normally horn colored but ranges from light to dark brown. (the same as most freshwater snails). Encrusted shells can be any color.
Size: Almost all western populations reach a maximal size very near 5 mm. One population in Idaho (Cassia Creek of the Raft River) regularly pushes 6 mm. Reports of Potamopyrgus antipodarum reaching 12 mm do not apply to the western USA.
Shape: The shell is rather elongate compared to most western species. Like most snails, it is dextral (opening to the animals right). A full grown shell normally has 5 or 6 whorls, which is higher than most western species.
Keel: In some western populations, a weak keel is present about mid whorl in some to most individuals (last picture above). Many populations lack this keel entirely (first 3 pictures above). This keel is not present on any native western snail species.
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Identification is greatly substantiated by noting that New Zealand mudsnail populations are all female and the snails are live bearers. Males are present only as an extreme rarity in western North America (see the photo gallery for pictures of a relaxed male. The developing young form a major part most adult snails (first 2 pictures above). The embryos are normally well developed in the summer and fall, but may be pretty rudimentary in the winter and spring. If eyes and a coiled body are present, it is an embryo, not an egg. Other live-bearing western snails include: Tryonia (widespread in the southwest, south of Great Salt Lake), Eremopyrgus (limited to a few warm springs in the Steptoe Valley, Nevada) and Melanoides (introduced into scattered warm springs). The newly released snails are about 1 mm long and often look like sprinkled black pepper on the substrate. As a livebearer, the young snails are never present without some adults. See the photo gallery for more pictures of the embryos.