Mouse feces are found in areas where mice are most active, near feeding, breeding, and nesting areas. Droppings are black, cylindrical in shape, and between three to six millimeters (about 1/4 inch) in length. Mouse droppings resemble small grains of rice and are commonly mistaken for cockroach droppings. Always wear protective gloves when disposing of mouse droppings; in heavily infested areas, also wear a respirator. Don’t sweep or vacuum the droppings—disturbing them could release bacteria and virus particles. Droppings should be carefully picked up and disposed of in sealed plastic bags.

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Strange scratching or scrabbling noises in the walls, especially at night, are another sign of mice. Mice are excellent climbers and jumpers, and are capable of fitting through openings much smaller than their bodies; they can use the spaces in between joists to travel from one part of the house to another. Mice are typically nocturnal, so you will probably hear them only at night.

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A regular pattern of dirty smudges along floors and baseboards could be a sign of mouse traffic. Mice tend to run along walls and follow the same route night after night. Over time, the dirt from their fur and feet will create a distinctive streaking or smudging tracery along the path. You may also find small patches of urine or droppings along the route.

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If you find unexpected holes, tears, or signs of gnawing in bedding, clothing, insulation, fabrics, or other materials, it is a good bet that mice are present. Mice use shredded materials to build nests, which are usually located in dark corners, drawers, cabinets, or storage boxes, or behind fridges and other large, seldom-moved appliances. Mice will often chew small holes in the bottom of drawers, cabinets, and furniture and fill the holes with shredded material.

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Mice have teeth that grow continuously, so they need to gnaw on hard, inedible materials, including wood, plastic, cables, and electrical wiring, to wear them down. This makes mice a serious fire hazard, as they can chew through shielding and wiring, which can cause shorts, power outages, equipment damage, or fire.

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Mice have a distinctive, musky ammonia smell that smells like stale urine. This odor can be particularly noticeable in enclosed areas, such as pantries, cabinets, or drawers. The smell may also be evident along baseboards and walls where mice frequently travel; the odor helps them establish and mark their territory.

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One adult mouse can consume between three and four grams of food per day, and mice like many of the same foods we do: cereals, grains, seeds, nuts, fruits, meats, chocolate, and candy. Mice especially like foods that are high in fat, such as bacon, cheese, and butter. They'll make up to 30 visits a day to an identified food source, such as a pantry or cupboard. Strange holes or tears in packages are indications that mice may be raiding your pantry. If you think you've been sharing food with mice, throw it away—food that has been contaminated by mice may cause dangerous illnesses in humans, including salmonella, hantavirus, and fever.

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Finding a dead mouse in the house—or, heaven forbid, seeing a live one—is a sure sign that you have a problem. Seeing a mouse out and about during the daytime can indicate a particularly heavy infestation, one that might require the services of a trained pest-control professional to eradicate.

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