The three commensal rodents may be separated from native rodents by the character of their tails. Tails of commensal rodents are scaly and sparsely covered with hair; tails of native rodents have more hair and smaller scales. Fur color varies widely among rodents, even between individuals of the same species. Therefore, color alone is not a good identifying characteristic.

Field identification characteristics include relative length of tail and size of body and ears.  A roof rat's tail usually is longer than its head and body.  When the tail is stretched over the back it extends past the nose.  The Norway rat's tail, however, does not extend past its nose when stretched over Its back.  A roof rat's ears also are large and conspicuous.

The roof rat's eyes are larger and set farther forward than the Norway rat's.  The Norway rat's body is heavier and stockier when compared with the roof rat which has a pointed muzzle and a slim body.  Juvenile rats are distinguished from adult house mice by their larger feet, heads and tails.