The three main types of thermometers are (1) a digital thermometer, (2) an ear thermometer, and (3) a temporal artery thermometer. You should not use the old mercury thermometers anymore, if you still have one in your home.
A fever is a temperature over 100.4 degrees F, or 38 degrees C.
For newborn infants under 3 months of age
Use a digital thermometer and measure a rectal temperature. Any digital thermometer can be used rectally; there is not a specific “rectal” thermometer. This is the most accurate way to measure core body temperature, and it is important to have an accurate temperature in this age group. Measure your baby’s temperature if he/she feels warm or is acting sick. Signs of illness in this age group may include lethargy, poor feeding, or irritability.
For infants over 3 months of age and older children
You may use any type of thermometer you prefer. While a rectal temperature is still the most accurate, fevers in this older age group are not as potentially serious and it is not necessary to continue taking rectal temps. Digital thermometers may be used orally, in the armpit, or rectally. Ear thermometers measure temperature in the ear, and temporal artery thermometers measure temperature on the forehead.
For more instructions on how to take your child's temperature, please check out this article on BabyCenter.com.