There are a few signs that can indicate if a bird is sick. Some common signs include changes in eating or drinking habits, decrease in activity levels, changes in droppings (color, consistency), breathing difficulties, sneezing, discharge from eyes or nostrils, feather puffing, or changes in vocalization. If you notice any of these signs in a bird, it is recommended to consult with a veterinarian who specializes in avian care for a proper diagnosis and treatment.