Mice are active, alert animals. When healthy, their eyes and noses are free of discharge, their coats are sleek and shiny, and their droppings are firm and elongated.
Unhealthy mice may have a dull coat, eye or nasal discharge or diarrhea. They can also be lethargic, aggressive or sleep almost constantly.
If you see any of these signs in your mouse, take it to a small mammal exotics veterinarian immediately.