The best way to tell if a cat has a fever is to take its temperature and see whether or not it falls in the normal temperature range for cats, which is 100.4 to 102.5 degrees F, according to WebMD. If a cat's temperature rises above 106 F, it can suffer damage to its organs and needs to see an emergency vet immediately.
Fevers in cats are generally caused by sickness or injury. The immune system can activate a fever if a bacterial infection, viral infection, fungal infection, tumor, trauma injury, certain medication or disease is present in the cat. If a fever appears for more than two weeks, and there is no apparent reason for the fever then it is called a FUO, or fever of unknown origin.
A person can see a fever in a cat by looking for the telltale signs of a fever. Most cats that have a fever will have one or more of the following symptoms: depression, appetite loss, lack of energy, lethargy, not drinking enough, not grooming themselves, shivering or breathing quickly in shallow breaths. A cat may also have diarrhea, vomiting or sneezing. The only way to determine whether or not a cat has a fever for sure is to take its temperature, which can be done at home or at the vet's office.