1a. The top surface of the slug is uniform in pattern, not appearing to be divided into front and back parts (Philomycidae and Testacellidae; the mantle covers most or all of the slug’s top surface).
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1b. The top surface of the slug is apparently divided into two parts – front and back (Arionidae, Limacidae, Milacidae; the slugs whose mantle covers only part of the top).
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2a. The top surface is heavily mottled or spotted, adults are often large (more than 5cm).
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2b. The top is blue-gray, tan or very lightly speckled, and adults are often small (5cm or less; Pallifera species).
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2c. The top is pale, with a tiny shell on the posterior (Family Testacellidae).
Testacella haliotidea – earshell slug (non-native)

3a. The fringe or “sidewall” of the foot is pink or orange and the mucus is orange (these are much easier to see in live animals). The top surface has one or two close gray-black central stripes.
Philomycus togatus – toga mantleslug

3b. The foot fringe is olive or gray (this is a dark color that runs the entire length of the animal – don’t be fooled by a dark tail tip, which is not diagnostic).
Megapallifera mutabilis – changeable mantleslug

3c. The foot fringe is pale.
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4a. There is a central mottled stripe on top, plus a mottled stripe on each side (the top stripe may sometimes appear as two very close stripes).
Philomycus flexuolaris – winding mantleslug

4b. There are two sets of dark, elongated spots along top, superimposed upon a paler, broad central stripe.
Philomycus carolinianus – Carolina mantleslug

5a. The slug has a uniformly pale foot. There is a gray back, with a speckled center line most distinct near head.
Pallifera dorsalis – pale mantleslug

5b. The sole of the foot has two narrow blood-red lines running lengthwise along edge, starting from front. The animal has a plain tan or gray back.
Pallifera ohioensis – redfoot mantleslug

6a. The slug’s top surface has a “keel” – an acute top edge – on at least the posterior part of the back. Posterior end of tail is sharp in top view and lacks a mucus pit. A “fingerprint” channel pattern on the mantle may be present or absent (Family Milacidae, Family Limacidae).
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6b. The slug has a keel only on the tip of the tail. There is no “fingerprint” channel pattern on the mantle (Family Limacidae, most).
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6c. The slug has a rounded back with no keel. Posterior end of tail is rounded in top view with a conspicuous posterior mucus pore. There is no “fingerprint” channel pattern on the mantle (Family Arionidae).
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7a. The keel extends from the tail all the way to the mantle. A horseshoe-shaped groove is found on the mantle. Mantle lacks “fingerprint” channel pattern (Family Milacidae).
Milax gagates – greenhouse slug (non-native)

7b. The keel does not reach all the way to the back of the mantle. Mantle lacks a horseshoe-shaped groove. A “fingerprint” channel pattern is found on the mantle (Family Limacidae).
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8a. The animal’s top surface is dark or pale, and may be speckled; adults are medium-sized (extended length 3.5 to 5 cm). Exudes milky mucus when irritated. Breathing hole on right side with a pale border.
Deroceras reticulatum – gray fieldslug (non-native)

8b. The top surface is dark-colored and adults are small (extended length 1.5 to 2.5 cm; often fast-crawling). Mucus colorless even when irritated. Breathing pore on right side without a pale border.
Deroceras leave – meadow slug

8c. The top surface is heavily mottled on front, with a mottled stripe or uniform color on the back part; adults are very large (10 to 20 cm).
Limax maximus – giant gardenslug (non-native)

8d. Mantle and back with bands, not blotches; medium sized (5 to 7 cm).
Limax valentiana – threeband gardenslug (non-native)

9a. The slug has orange color on its body.
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9b. The slug is without orange color on its body (though it may be pale yellow).
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10a. Orange is found on the side both above and below a dark side stripe. The mucus is orange (to see the mucus color best, smear the slug against white paper). Adults are up to 7cm.
Arion subfuscus – dusky arion (non-native)

10b. Orange is found only below the dark side stripe, the sole of the foot is pale. The mucus is clear. Adults are up to 5cm.
Arion fasciatus – orange-banded arion (non-native)

10c. There is a dark side stripe, but no orange on the side. The sole is orange or yellow, but the mucus is clear. Adults are up to 4cm.
Arion hortensis complex – garden arion (non-natives); this is a complex of three species – Arion hortensis, Arion distinctus, and Arion owenii - that are best told apart by dissection

11a. There is a dark side stripe. The back is gray-yellow back and the sole nearly colorless. There are no pronounced bumps on the tail. Adults are up to 4cm.
Arion circumscriptus – brown-banded arion (non-native)

11b. The side stripe is faint or absent. The back and sole are pale yellow. There are pronounced bumps on the tail when the animal is contracted. Adults are small, only up to 2cm.
Arion intermedius – hedgehog arion (non-native)