Cats are experts at hiding signs of illness or injury, so it's important to pay attention to any changes in their behaviour, habits, or appearance and seek veterinary care if you have concerns.

Here are some common things to base your observations on:

1. Changes in appetite: Cats can be picky eaters, but a sudden loss of appetite or increased appetite can be a sign of an underlying health issue.

2. Changes in behaviour: If your cat becomes more lethargic, irritable, or withdrawn, this may indicate that something is wrong.

3. Changes in litter box habits: If your cat starts urinating or defecating outside the litter box or has difficulty using the litter box, it could be a sign of a urinary tract infection, bladder stones, or other medical conditions.

4. Vomiting or Diarrhoea: Occasional vomiting or diarrhoea is normal for cats, but if it becomes frequent or persistent, it may indicate an underlying health problem.

5. Difficulty breathing: If your cat is panting, wheezing, or struggling to breathe, seek veterinary care immediately.

6. Unusual discharges: Any unusual discharges, such as discharge from the eyes, nose, or ears, may indicate an infection or other medical condition.

7. Changes in coat condition: If your cat's coat becomes dull, dry, or matted, it may be a sign of an underlying health issue or poor nutrition.

8. Unexplained weight loss or gain: Significant weight loss or gain without changes in diet or activity level can be a sign of illness.

9. Lumps or bumps: Any unusual lumps or bumps on your cat's body should be evaluated by a veterinarian.

10. Changes in drinking habits: Increased or decreased drinking can indicate an underlying health problem such as diabetes, kidney disease, or hyperthyroidism.