Signs of a Healthy Chicken
Eyes:
Chicken eyes should be clear and shiny. When a chicken is alert and active, its eyelids shouldn't be showing. You shouldn't see any discharge or swelling around the eyes.
Nose:
Both nostrils should be clear and open, with no discharge from the nostrils.
Mouth:
The chicken should breathe with the mouth closed, except in very hot conditions. If cooling the bird doesn't make it breathe with its mouth closed, it is ill.
Wings:
The wings of chickens should be carried close to the body in most breeds. A few breeds have wings that point downward. (You need to study breed characteristics to see what is normal for your breed.) The wings shouldn't droop or look twisted.
Sometimes droopy wings are a sign of illness in the bird. A damaged wing that healed wrong won't affect the laying or breeding ability of the bird. However, some birds are hatched with bad wings, which is usually the result of a genetic problem. These birds should not be used for breeding.
Feathers:
In general, a chicken shouldn't be missing large patches of feathers. Hens kept with a rooster often have bare patches on the back and the base of the neck near the back. These patches are caused by mating and are normal. You should never see open sores or swelling where the skin is bare.
Sometimes feathers are pulled out, particularly tail feathers, when capturing a bird. If the bird appears healthy otherwise and the skin appears smooth and intact, it's probably fine.
A healthy bird has its feathers smoothed down when it is active. Some breed differences are noteworthy — for example, obviously, a Frizzle with its twisted feathers will never look smooth. A bird with its feathers fluffed out that isn't sleeping or taking a dust bath is probably ill.
Feet and toes:
The three front toes of chickens should point straight ahead, and the feet should not turn outward. The hock joints shouldn't touch, and the toes shouldn't point in toward each other. Chicken feet shouldn't be webbed (webbing is skin connecting the toes), although occasionally webbed feet show up as a genetic defect. You shouldn't see any swellings on the legs or toes. Check the bottom of the foot for swelling and raw, open areas.
Vent:
The feathers under the tail of the chicken around the vent, the common opening for feces, mating, and passing eggs, should not be matted with feces, and you shouldn't see any sores or wounds around it.
Mental state:
The chicken should appear alert and avoid strangers if it is in a lighted area. Birds that are inactive and allow easy handling are probably ill. Chickens in the dark, however, are very passive, and this is normal.
Activity Level:
Here again, differences exist between breeds, but a healthy chicken is rarely still during the daylight hours. Some breeds are more nervous and flighty; others are calm but busy. In very warm weather, all chickens are less active.
*information from chicken health for dummies (2nd edition - Kimberly Willis, Robert T. Ludlow)
*Please note these are general guidelines. Consult your vet for more detailed information and guidance.