Question: how to know if there are rats in the walls?
Answer: Squeaking or scurrying sounds in the walls.
Running or soft footstep sounds primarily at night.
Piles of droppings in an area behind a stove, in the basement or attic, or on the ground.
Food packages, containers or wooden spoons that have been gnawed on.
Rub marks, which look like an accumulation of dirt and grease.