Dogs with fevers will usually let you know they are feeling sick; they feel lethargic, inappetent, and all around crummy. Sometimes they will feel warm to the touch, but a fever should never be diagnosed purely based on how a dog feels to the touch. Your veterinarian can confirm the presence of fever and help determine the underlying cause.
Diagnosis almost always includes blood and urine testing, and in some cases imaging like x-rays or ultrasounds. Commonly, fevers are caused by infections and require antibiotic treatment. Others are caused by inflammatory or immune mediated conditions that require steroids. In almost all cases, some degree of supportive care is needed; this includes fluid therapy, pain medication, and nutritional support.
The best thing you can do if you think your dog has a fever is to seek prompt veterinary care. Getting a diagnosis can take some time. Supportive care can be initiated until the definitive cause and treatment are determined.
Remember, do not confuse fever with hyperthermia (your dog being overheated from external factors, like overexertion or a hot car). Dogs with fevers cannot be made to feel better by simply cooling them down. Also, importantly, never give over-the-counter medications like Advil or Tylenol to a dog with a fever.