1. Rodent Droppings
Fresh rodent droppings are moist and dark. As they age, they end up drying out and will easily crumble if touched. You are most likely to find near food packages, under sinks, in drawers or cupboards, in places that are hidden from sight, and along rodent runways. Most rodent droppings will be found close to where the rodents are feeding or nesting, so you'll need to check the areas close to any new-found droppings to determine if there is still an active infestation.
2. Gnaw Marks
Entirely contrary to droppings, gnaw marks which are new will appear as a light color, while older ones will become dark. Such marks will usually be found on the structure of your home or on food packaging. Gnaw marks can also show you whether you have an infestation of rats or mice in your home. The bigger they are, the more likely it is that they were caused by rats, since mice only tend to leave very small gnaw marks.
3. Foul Odors
Family pets often get pretty excited in places where rodents are present due to the particular odor they give off. If your pet is pawing somewhere it never expressed interest before, then you should use a flashlight to inspect the area for mice or rats. If you've got a large-scale infestation on your hands, then you may also be able to smell a stale odor coming from the problem area.
4. Tracks and Runways
If rats or mice are currently present in your home, their tracks and runways are likely to be pretty distinctive, becoming fainter as time goes by. Runways or tracks can be detected easily with a flashlight or a blacklight. You might see smudge marks, urine stains, footprints or droppings. If you suspect an area has been infested, place a thin layer of baby powder or flour there. If your suspicions were correct, you will end up seeing their trails in the powder later on.
Rodents tend to make use of materials such as fabric, shredded paper or dried plant matter to construct their nests. If such materials are found, along with any of the other signs mentioned above, then it is highly probable that there is an infestation of rats or mice in your home.