For starters, you'll need a good digital thermometer. Digital thermometers are easy to use, easy to read, and fast — most give an accurate reading in ten seconds to two minutes. Some digital thermometers are specifically designed for use in the rectum, but most can be used rectally, orally, or under the arm. Whenever you think your baby has a fever, take his temperature with a thermometer. Feeling the skin or using temperature-sensitive tape is not accurate, especially when the infant is experiencing a chill.

The American Academy of Pediatrics no longer recommends mercury thermometers because these glass thermometers may break and, as their mercury vaporizes, it can be inhaled, resulting in toxic levels.

Where to put the thermometer to get the best reading?

The best type of thermometer — or the best place to insert the thermometer, in some cases — depends on your child's age.

Birth to 3 months. For newborns, use a regular digital thermometer to take a rectal temperature. To take the temperature in your baby’s bottom, turn on the digital thermometer and then put a small amount of lubricant, such as petroleum jelly, on the small end of it. Place your infant across your lap or something firm, either faceup or facedown (if he’s facedown, put one hand on his back; if he’s faceup, bend your child’s leg to his chest, resting your free hand on the back of his thighs). Then gently insert the small end of the thermometer in your baby’s bottom (rectum), putting it in about ½ inch to 1 inch. Hold the thermometer in place for about one minute or until the device signals that it’s done (by beeping or lighting up). Remove it and read the number.

3 months to 4 years. For older infants and toddlers, you can still use a regular digital thermometer to take a rectal temperature or an armpit temperature. You can also use a digital ear thermometer, called tympanic thermometers.

• Digital ear thermometers are usually more expensive and can only be used in the ear. Ear thermometers are generally quick, safe, and comfortable. The only problem is that if you don't insert the ear thermometer exactly right, it can be hard to get an accurate, consistent reading. Ear thermometers aren't recommended for babies younger than 3 months because little babies' narrow ear canals make it hard to insert the sensor properly.

• 

  For an armpit reading, undress your baby from the waist up and cradle him or sit him on your lap or next to you. Try to keep him relaxed and occupied by feeding him or showing him a book or a toy. Make sure your baby's underarm area is dry, and then slip the bulb of the thermometer into his armpit. The bulb needs to be in full contact with his skin, so hold your baby's arm firmly against his side or bent and folded across his chest. When the thermometer beeps, take it out and read the display.

4 years and older. By age 4, most kids can hold a digital thermometer under the tongue for the short time it takes to get a temperature reading. If you're taking your child's temperature orally, place the end of the thermometer under his or her tongue, towards the back of the mouth. Have your child close his or her lips on the thermometer. Tell your child not to bite down or talk. When the thermometer beeps, remove it and check the digital reading. If your child has been eating or drinking, wait at least 15 minutes to take his or her temperature by mouth. If your child is too congested to breathe through his or her nose, you may need to take an armpit or rectal temperature — or use a digital ear thermometer.

Can be used by children between 3 months and 2 years of age. They require that your baby keep the pacifier portion in his mouth for about three minutes. Pacifier thermometers results’ are a bit low.

Temporal artery thermometer.

Designed to measure the temperature of the skin surface over the temporal artery, a major artery of the head. The reliability of temporal artery thermometers hasn't yet been verified.