The best-known rat species are the black rat (Rattus rattus) and the brown rat (Rattus norvegicus). They are similar to mice, but are easy distinguished from these rodents by their size. Rats can be two to five times larger than a common house mouse.
The common species are opportunistic survivors and often live with and near humans; therefore, they are known as commensals. The average lifespan of any given rat depends on which species is being discussed, but many only live about a year due to predation.
Droppings, typically left behind in kitchen cabinets, pantries, cupboards, drawers, bins, and anywhere else they think they might find food, or where they scurry to avoid predators. Rodents are prolific poopers, so it's pretty easy to spot if you have an infestation. It's also not uncommon to see droppings along walls, on top of wall studs or beams, near nests, and in boxes, bags, old furniture, and other objects.
Squeaks and other noises. Rodents aren't what you'd call quiet. If they're in your house, you'll hear squeaks, rustling, and scampering sounds as they move about and nest. Noises are often more apparent at night as you're going to bed and they're waking up.
Urine pools or trails. Rodents are notorious for having weak bladders, and they'll dribble all over the place. House mice sometimes make things called "urinating pillars," which are small mounds consisting of grease, dirt, and yes, urine. Sometimes you'll see tiny drops of urine leading to a mound.
Nibble marks on food boxes, food, or containers. These telltale signs are often accompanied by nearby droppings.
Nests. Rodents build nests from soft, fuzzy, or warm materials, such as fabric, furniture stuffing, quilt batting, shredded paper, grass, and twigs, and will typically stuff them into sheltered, out-of-the-way places like boxes, cabinets and closets, walls, even the subspace between ceilings and floors. Other possible mouse nest sites include dressers, behind and inside appliances, and machinery, even computer cases — basically, anywhere it's cozy and warm.
Grease marks. Mice can wedge through openings as small as a quarter of an inch in size. As they do, they often leave greasy smears — caused by oil and dirt in their coats — behind. The marks left by mice are fainter than those left by rats. If you find large greasy smears, you should suspect a rat infestation instead.
Gnaw marks. Gnawing is a defining characteristic of all rodents. They do it to keep their incisor teeth, which grow continually, in check. Wood is a favorite,but they'll pretty much chew on whatever suits them. This includes electrical wire, which, as noted in Electrical Fires, makes them a leading cause of structural fires. On wood, newer gnaws are light colored. They turn darker with age. Sometimes you won't see gnaw marks, but you'll see what looks like fine wood chips or coarse sawdust, especially along baseboards, door and window frames, and cabinets.
Holes in food packaging. Rodents will nibble into anything they can smell, including boxes and bags of pasta, rice, beans, and grain products. Dog food bags are also prime-time rodent magnets, and especially so for rats, who like the meaty smell as much as canines do. Another popular nibble, although not a food product: soap.
An "off" aroma, or smell. House mice have a distinctive musky odor. It's hard to describe, but once you smell it, you'll never forget it.
Tracks. Look for footprints or tail marks in dusty spots. The type of track and tail marks can tell you what kind of rodent you're battling. Mice have the smallest feet, measuring 3/8 inch or less. Rat tracks average between 3/4 to 1 inch. Rats also drag their tails, which leaves a mark between their feet tracks. If tracks are hard to spot, shining a flashlight across a suspicious area can help illuminate them.
Pet excitement. If Rover or Miss Kitty is acting a bit nuts (more nuts than usual?), especially around a possible mouse hiding area, chances are good a critter has been there or is still there.