How do I know when I really have a fever?

Anything above 37 deg C used to be considered a fever, for temperature taken by mouth.
But your temperature varies throughout the day from 36.2 to 37.2 deg C.
So going 0.1 or 0.2 deg C above 37 deg C is insignificant.
To avoid over-diagnosing people as having a fever, the Health Promotion Board has reviewed what constitutes a fever and has decided the following, as at yesterday.

A adult has a fever when the thermometer reads:
- 37.0 and above (armpit temperature)
- 37.3 and above (oral temperature)
- 37.7 and above (ear temperature)

A child has a fever when the thermometer reads:
- 37.3 and above (armpit temperature)
- 37.6 and above (oral temperature)
- 37.9 and above (ear thermometer)

Children's temperatures are slightly higher because they have a higher metabolic rate than adults. Your temperature varies at different parts of your body.
The ear is the highest, followed by the mouth, forehead and armpit.

Regardless of where you take your temperature, it is important to use the right technique recommended for your thermometer in order to obtain an accurate reading.
Each person also has a unique baseline temperature, normal for him or her, and it may be the lower or higher end of range of 36.2 deg C to 37.2 deg C It helps if you and your family take your temperature at different times of the day to determine what each person's baseline temperature is.