I was curious, too, since you mentioned it. I didn't know, but I found this:
When your dog is ill, you may have to determine whether or not he has a fever by taking your dog's temperature. It's relatively easy and all you need is a thermometer. Learning how to take your dog's temperature properly can help determine if immediate veterinary care is needed.
Your dog's normal rectal temperature is 100.5 to 102.5 degrees Fahrenheit. Higher temperatures may necessitate a trip to your veterinarian, depending on other symptoms. Feeling the ears, nose or head is not considered a reliable method; you have to determine your dog's internal temperature to find out for certain. This is done using an oral or rectal thermometer, either digital or mercury. Ear thermometers can also be used in dogs. They are generally fast and easy but it is essential to use a proper technique to obtain an accurate temperature reading.
How to Take Your Dog?s Temperature