You can measure most animals' temperatures rectally. Start by lubricating the thermometer's tip using baby oil or petroleum jelly. Gently raise your dog's tail, then carefully insert the thermometer probe an inch deep into your dog's rectum. You insert it in a little more, perhaps two inches for larger breeds. Hold the probe in place until it beeps or you attain a reading.
Have another person hold and comfort your dog while taking their temperature because most dogs don't enjoy the process. Quick-acting thermometers also simplify the process. Although taking a dog's temperature is not enjoyable for either of you, it is necessary to determine the severity of the dog's sickness and the level of care it needs.