This is the question we get asked most on the phone, and it is worth answering properly, because the two animals need different work. A mouse problem is a kitchen and wall-void job. A rat problem is a crawlspace, roofline and exterior job. Traps set for the wrong one catch nothing.
You can usually settle it in five minutes without ever seeing the animal.
1. The droppings
This is the reliable check. Size and shape separate all three of the rodents we deal with across Metro Vancouver.
A rougher version of the same rule: mouse droppings look like grains of rice, rat droppings look like raisins. If you are finding a handful of large ones you have rats. If you are finding a hundred small ones you have mice, and have had for a while, because one mouse produces a great many.
Do not dry-sweep or vacuum them. Deer mice in this region carry hantavirus, and the risk comes from breathing dust off dried droppings that have just been disturbed. Damp them down first, wear gloves, and bag them.
2. Where the noise is
Height tells you the species nearly as well as the droppings do.
Overhead, in an attic or a top-floor ceiling, starting at dusk, is usually roof rats. They are light, they climb well, and they use hedges, wires and branches to reach a soffit without touching the ground.
Low down, in a crawlspace, under a shed, along a foundation, is Norway rats. Heavier, and diggers. Sewers, compost, blackberry and the space under a deck are where they live before they find a way under the house.
Noise inside a wall on more than one floor usually means they are travelling in the framing. In an older Vancouver or New Westminster house they can do that from bottom to top.
Mice are quieter and quicker. Scratching behind a kitchen cabinet at night, and often nothing you can hear at all.
3. Grease marks and gnaw marks
Rats leave a dark greasy rub mark where the same route gets used every night. Along a joist, over a pipe, at the edge of a wall. Mice leave very little.
Gnaw marks say the same thing by size. A mouse cuts a hole in drywall about the size of a dime. A rat cuts one closer to a toonie, with rougher edges. Chewed wiring, a chewed dryer hose or a hole through a plastic pipe is rat work, and a mouse rarely gets that far.
4. What has gone missing
Mice shred. Insulation pulled into a nest, cardboard and paper reduced to fluff, usually somewhere warm, which in most houses means next to the hot water tank.
Rats carry things off. Dog food moved out of a bag and into a cache under the deck is a rat, and it is one of the clearer signs, because a mouse mostly eats where it finds food.
Why the answer changes the job
For mice the work is interior. Find the runs, seal the gaps, set snap traps tight against the wall on the route, and deal with whatever is feeding them. A mouse gets through a gap the width of a pencil, so the sealing list is longer and finer than it is for rats.
For rats the work is mostly outside. Crawlspace vents, service penetrations, the roofline, the branches touching the house. The interior part is small. The exterior part is the entire fix.
For both, sealing comes before trapping. Otherwise you are managing a population instead of ending one, which is covered in more detail in the autumn rodent post.
One more thing worth knowing. A single mouse sighting is rarely a single mouse, because they breed fast enough that by the time one crosses a floor in daylight there is a nest. A single rat sighting genuinely can be one rat, particularly in autumn while they are on the move.
Or have someone look
Identification is the first thing a technician does on site. You get the species named, the entry points found, and a fixed written price before anything is applied.
The rodent guide has the full species list, the signs and the twelve-month activity curve. There is also a two-question identifier on the pest library page if you would rather narrow it down yourself first.