Beyond specifically following the instructions on your thermometer, here is advice from doctors on getting accurate readings on possible fever:
Use a digital thermometer. Medical associations no longer recommend glass thermometers filled with mercury, an environmental toxin. If you have an older thermometer, contact your local health department about disposing of it safely.
Time it right. Don’t check for fever soon after eating something hot or cold. Hot baths, layers of warm clothing and vigorous exercise also can throw off readings.
Tips for oral readings: Keep the thermometer under your tongue toward the back of your mouth. Use your lips to close your mouth around it — don’t bite down on it. Breathe through your nose and don’t open your mouth to talk.
Rectal readings: Place the child on her stomach on your lap or a flat surface such as a diaper changing table. Rub a little petroleum jelly on the tip of a rectal thermometer and insert it 1/2 to 1 inch inside the rectum (never push if you meet resistance).
Ear readings: If a thermometer comes with plastic covers, dispose them after a single use. Gently place the thermometer into the entrance of the ear canal; again, don’t push.
Underarm readings: Make sure the child’s armpit is dry and take off his shirt off so only skin will touch the thermometer.
Recognize other signs of fever. Those include flushed cheeks, chills, muscle pain and headaches. Also note that body temperature tends to be higher at night than in the morning.
Don’t just look at fever. When considering how sick someone is, note changes in eating and sleeping habits, alertness and mood. All can help you decide if you need a doctor.