A healthy chicken should have a comb which is red, plump and glossy and her keel should be well padded with flesh either side.
Her vent should be nice and clean, her crop should fill up when she eats and slowly deflate afterwards (if it stays firm it may be impacted). Most importantly she should be active, eating, drinking well, preening and scratching.
Like any other animal they require the correct environment to live in with sturdy, well designed shelter to escape the elements as well as to provide protection from potential predators. Locking them up at night is a good way of preventing your brood being taken by a fox!