You can usually hear rats, and you’ll typically hear them at night. You’ll most often hear them moving in the attic or on the roof. You may hear scratching or scampering noises. 

You can also check the attic itself for droppings. You’ll tend to see a lot of them. You might see dark, heavy stains in pathways that rats are traveling frequently. Droppings look like little grains of dark brown rice to twice that size. 

Rats have very oily bodies and they rub against certain surfaces as they travel, leaving oily, unpleasant stains. If you’re seeing them in open areas then the swarm has grown very large and you need immediate help. 

If the rat swarm grows too large, they might grow bold enough to start entering areas humans frequent, and you’ll start seeing them face to face.