Like most of us, rats are creatures of habit. Once they find a good path to a watering hole or food source, they’ll follow it over and over as they trek back and forth. The fact that they have poor eyesight also contributes to this.
This repetition will eventually lead to little rodent paths. Watch for dark, greasy smudges on walls, floorboards, between stair banisters, and anywhere else that rats might pass through.
If you suspect that you’ve found a rat trail, try leaving a tiny bit of flour or baby powder at the site. The next time the rats come through, they’ll leave track marks confirming their miserable existence.