The Norway or Brown Rat, the most common pest rat, has a freckled tail. A house mouse, the most common pest mouse, has a nearly uniform-color tail.
The Black Rat or Roof Rat, the second most common pest rat, has a black tail that is longer than its head and body combined. [The Brown Rat's tail is shorter than head and body combined, and freckled.] Black Rats are not always black; they are very agile. Brown Rats are agile, but not as agile as Black rats, which in L.A. are known to nest in the tops of palm trees.
The Harvest Mouse is so tiny that even the adults cannot be mistaken for juvenile rats. They also have plain tails and are very agile.