Lethargy and Sluggishness

A dog who has an infection is likely to be very lethargic, sluggish and to seem uninterested in his regular activities. Typically he will no longer want to play, go for walks or ride in the car. He's more likely to prefer to sleep most of the time and may hide in an out-of-the-way spot so that he's less likely to be disturbed. Suspect an infection in any dog who was active and has become lethargic. Also consider infection in an older dog who seems less energetic than usual.
Running a Fever

If your dog has an infection he will likely have a fever, though his temperature may be normal with a localized infection such as a bite wound or other injury. The normal temperature for a dog, typically taken rectally, is between 101 and 102.5 degrees. A temperature higher than 103 degrees is considered to be a fever and may indicate an infection. Unless there is an obvious cause for the fever, such as a vaccination within the past day or two, take your dog to the vet.
Changes in Eating and Drinking

A loss of appetite may signal that your dog has an infection. A previously hearty eater may refuse his food or just nibble at it. He may even refuse his favorite treats if he's got an infection. Even if he's refusing food, your dog may begin to consume more water than usual, a possible sign of a urinary tract infection. Such an infection may be accompanied by more frequent urination, often foul-smelling, bloody or cloudy urine.
Other Indications

The specific signs of an infection depend a lot on the type and location of the infection. Some things to watch for that indicate your dog has an infection are the presence of pus or an abscess. If it's a skin problem, near the surface of the skin or in his ear you also may notice that he scratches or digs at the spot or that the skin is hot, red or swollen. Some infections will cause your dog to cough, with or without the presence of mucus; he may vomit, have an obvious nasal discharge or show other signs of illness. In all cases he should be seen by a veterinarian as soon as possible to prevent further complications.