The temperature of a healthy dog is 101-5 F (as against 98-4 in a human being), and the only reliable way in which to find out if your dog is ill is to take its temperature.
This is done by gently inserting a blunt-nosed clinical thermometer which has been smeared with vaseline into the dog's rectum, and holding it there for 3 minutes (the coating of vaseline prevents it registering more quickly). Get someone to hold the dog's head while you are doing this, as it is imperative that it should remain absolutely still.
The temperature can also be taken by placing the thermometer in the armpit, or in the groin. This will take longer and the reading will not be nearly so reliable as if taken per rectum, and is not really recommended.
If the dog's temperature is above normal, or indeed very sub-normal (say under 100)-for this is an equally dangerous sign-call a veterinary surgeon or take your dog, preferably by car or taxi, for it should not be exerted in any way-to his surgery, or to an animal clinic.