Unlike humans, the normal temperature for a dog is between 38.1 and 39.2 C. Use a digital thermometer, coat it with lubricant (with petroleum jel or glycerine), insert it in the dog's anus and wait for the time necessary. Remove it, clean it and read the result. Disinfect after every use and remember to use a thermometer only for your pet.
You may find this table useful:
36.7 C or less: hypothermia. Keep your dog warm and contact the vet immediately.
37.2 C: abnormal. Contact the vet.
Between 37.8 and 38.9 C: normal. 39.4 C: light fever. Call the vet for advice.
40 C: high fever. Call the vet.
40.6 C: danger. Cool down your dog and call the vet.
41 C or higher: heat stroke. Cool down your dog immediately and call the vet urgently.