The only way of knowing if our cat has a fever is by using a thermometer. Any other method is not flawless. Its temperature should be measured at the veterinary clinic from an early age. A grown cat will not let us do this easily.
The only way of measuring its temperature is through its anus. Once we insert it, we must angle it in order to touch its intestine wall. If not, we would be taking the temperature of its faeces.
We should always have a digital thermometer handy. It is better if it is a plastic one since the traditional crystal thermometer can be easily broken if our cat moves making internal small cuts which are quite painful.
Their normal temperature is higher than ours:
BETWEEN 38º-39ºC. IS NORMAL
This is a good way of controlling if our cat is ill. However, I know from experience that very few cats allow their temperature to be measured (as in the first picture). If it temperature is above 39ºC, we should go to the veterinary. Even if we don't notice any symptoms, it is evident that our cat is suffering a pathology which needs treatment.