While some people advocate feeling a dog's nose to determine if it has a fever, the only way to tell for sure if a dog has a fever is to takes its temperature using a rectal thermometer. It is best to use a digital thermometer made just for dogs. Lubricate the tip with petroleum jelly or baby oil, and insert it about an inch into the dog's rectum. Most digital thermometers provide a result within 60 seconds. 
A dog's normal temperature range is 99.5 to 102.5 degrees Fahrenheit. A reading of 103 degrees Fahrenheit or higher indicates that the dog has a fever.  A temperature of 106 degrees Fahrenheit or above can cause fatal complications. 
Fever often goes undetected in dogs. Some signs and symptoms that may indicate that a dog has a fever include:
- Appetite loss
- Discharge from the nose 
Infections, vaccinations and toxins are some things that can cause fever to develop in a dog, but sometimes the cause is unknown. A dog with a temperature over 103 degrees Fahrenheit requires a veterinary consultation, and a fever over 106 degrees Fahrenheit is considered an emergency. 
To lower a dog's fever, apply water to its fur, especially to the feet and around the ears. Encourage the dog to drink water to avoid dehydration.