Common Signs and Symptoms of a Sick Cat

General:

Bad breath
Change in appetite or water intake/output
Change in behavior or routine
Growling/hissing/inappropriate meowing or vocalization
Hiding
Limping
Sleeping more than normal, acting mopey or lethargic
Unable to sit or lie down comfortably. (Painful)
Weight loss

Digestive System:

Diarrhea
Vomiting

Hair coat/Skin:

Matted or oily hair coat/hair loss
Scratching/licking/biting at fur, skin, feet, ears
Visible wounds or lesions

Respiratory:

Breathing fast or shallow.  Breathing with mouth open.
Congestion
Coughing
Loss of or change in meow
Runny nose
Runny or glassy looking eyes
Sneezing
Squinting eyes or visible 3rd eyelids

Urinary:

Going to the litter box frequently
Straining/trying to urinate with no or little success
Urinating in places other than the litter box (look for blood)
Very small or very large amounts of urine in the box