Rats and mice may look similar, but they have a number of unique characteristics that help to set them apart:
Rats are, of course, bigger than mice. Some rats can measure up to 9 inches long (or up to 16 inches with their tail). Mice are typically 2-4 inches in length (without the tail).
Rats have larger feet, a thick and hairless tail, and small ears.
Mice, on the other hand, have larger ears and a long thin tail covered with hairs.
Rodent droppings differ as well. Rat droppings are shiny black and up to 3/4″ long. Mice droppings are smaller with pointed ends.