When choosing a bird you should look for an active, bubbly bird with clear eyes and nares (nostrils).
Its feathers should be clean and held close to the body. The signs of a sick bird are sitting quietly, alone in a corner or on the bottom of the cage, fluffed feathers, labored breathing, holding the wings out or down away from the body and any liquid leaking from the eyes or nares.
If you see a bird looking ill and it is caged with others, let the pet store or breeder know immediately so all the birds can be looked at by a vet and treated for any illness.
If one bird is sick chances are others will be soon.
Remember to take any bird you buy to an avian (bird) vet as soon as you can to get a check up. Yes, even birds need regular vet care!