The short answer for a mouse
A mouse health check is measured in moments, not minutes: clear eyes and nose, quiet breathing, a glossy coat, even front teeth, a clean tail, and quick, curious movement. Mice are tiny, fast, and easily stressed, so the best checks blend into normal handling, using a tunnel or tube he already trusts. Look for the changes mice actually show: sneezing or clicking breath sounds, fur chewed short by a cage mate, scabs around the face and shoulders, lumps under the skin, and a hunched, still posture in an animal who is normally everywhere at once.
Observe the group before singling one out
Watch the enclosure first, ideally during the active evening hours. A well mouse climbs, burrows, runs the wheel, and rears up to sniff, whiskers working constantly. Notice whether one mouse is being groomed until her coat looks clipped, a habit called barbering, or whether another is sitting apart, fluffed and squinting, while the group feeds. Check breathing from outside the bars: the flanks should move smoothly without exaggerated effort, and you should not hear repeated sneezing fits or chirping sounds. Look at food and water use and at whether droppings in the bedding look plentiful and firm.
Handling a very small patient
Let the mouse walk into a cardboard tube, then tip her gently onto your palm over the enclosure or a deep box so a jump lands safely. Never lift by the tip of the tail, and keep fingers loose rather than pinning her down. In your hand, glance at the eyes for cloudiness or discharge and the nose for crusting, then look over the coat for bald patches, redness, or small scabs, especially on the face, neck, and shoulders where scratching and cage-mate grooming concentrate. Feel with a feather-light touch along the sides and belly for lumps; in mice, new lumps are common enough that finding one early and having it assessed promptly is worthwhile.
Teeth, tail, and tiny details
Mouse front teeth grow continuously and should be even and unbroken; a mouse who drools, drops food, or loses weight may be struggling to eat, and that pattern deserves veterinary input. The tail should be smooth and clean along its full length, without kinks, sores, or rings of swollen skin. Look at the feet for sore spots and at the underside for a dirty or wet bottom, which is never normal for a mouse. Because mice weigh so little, a small kitchen scale gives useful trend information: weigh weekly in the same tube, tare the tube first, and record the result rather than trusting how she feels in your hand.
Escalate quickly with mice
Mice hide decline until it is advanced, so act promptly on laboured or noisy breathing, a hunched and inactive mouse, a head tilt or circling, refusal to eat, or a lump that grows visibly between checks. Bleeding, a prolapse, or a mouse found cold and unresponsive is an emergency. Vets who see small mammals can do a great deal more with an early visit and a clear history than with a late one, so bring your notes on weight, appetite, and when breathing or behaviour changed. Recording those brief weekly entries in KinsLe through the Pet Passport turns thirty-second checks into a history a vet can actually use.