Diagnosis
The veterinarian will obtain a thorough history of the cat's health from the owner, noting whether the animal goes outside often and its vaccination status.  A physical examination will reveal an abscess, which is either a firm or soft painful swelling, or an area of matted fur that may have pus oozing from it.  The veterinarian may extract a sample of the fluid from the wound and look at it under a microscope to positively diagnose an abscess.  Sometimes a culture will be taken to specifically identify the bacteria present and which antibiotics will be effective in killing them.