Detecting a fever in a dog can be difficult. Some signs to look for are lethargy and panting. If your dog does have a fever, he may also refuse his food. A dogs normal temperature at rest is between 99.5 and 102.5 degrees. This number can climb higher when your dog has some kind of medical problem. Anything higher than 104 degrees requires an immediate call to your veterinarian.The most common cause for fever in dogs is infection, particularly from a wound or abscessed tooth. The most accurate way to detect a fever in your dog is to use a rectal thermometer. Ask your veterinarian to show you how. Do not try to do it by yourself or never administer any pet medication with your vets advice.