Tips for discovering if your dog has a fever
If you touch Fido around the ears or the nose he may feel hot. Just as with humans a fever will raise the overall temperature to combat infections from bacteria, viruses and ingested toxins, the same is true of dogs. In humans touching the skin will ascertain if the temperature is raised. However, a dog's temperature is normally 3 degrees above that of a human: somewhere between 37.8C (100F) and 39.3C (102.7F). So Fido will naturally feel warm; but to a human touch how warm is warm? Fido is not in a position to speak.
Aside from the general listless malaise Fido may be showing other, more serious signs of illness: he may be scratching; he may have diarrhoea, or vomiting; he may find it difficult to move because of pain. If it is uncertain from touch just how hot Fido actually is, then the only sure method is by using a thermometer.
The best method is with a digital thermometer used rectally. It would be kinder to lubricate it with petroleum jelly and to distract Fido whilst this procedure is being carried out: Thermometers in one's personal little places are not pleasant, even for a dog.
Of course when in doubt it's always best to consult a vet, but if the temperature is slightly above 102F, and he's not showing any very serious signs of illness, then this might only be a mild fever. It may be considered wise to allow Fido to fight this off himself; it will boost his immune system. Unfortunately, as the temperature is being taking rectally, poo poo lurking in his back passage could give a false reading. Take his temperature at regular intervals, particularly after he's evacuated his bowels. If his temperature increases, or was already at 105 or above, Fido is a very ill dog and will need immediate medical attention.