A healthy bird will be alert and active, have bright and shiny eyes, clean feathers, clean nares (nostrils) and cere, be hungry and outgoing. Its diet should be varied - seeds should have composed less than 20% of its diet. Ideally, a balanced pelleted diet such as can be found here: www.harrisonsbirdfoods.com or here: www.lafeber.com/pet-birds should be fed as well as hard boiled egg yolk, pancakes and cornbread, the tops of fresh greens, dairy products such as yogurt and cheese, fresh fruits such as apples, pears, melon, kiwi, and berries, vegetables such as broccoli, carrots, beets, asparagus, cabbage, sweet potato, and squash, and even tiny pieces of meat. A poor diet is the most common cause of problems in pet birds!
Look at its living conditions - in addition to seeing what has been offerred as its diet, look at its droppings - the three parts of the droppings should be readily distinguished one from the other - the feces, urine, and urates.
You don't want to see a fluffed bird, a red/inflamed cere, cloudy eyes, open mouthed breathing, dirty feathers, tail bobbing, or no interest in food. You don't want to see a bird separated from the rest of the flock self-sequestered away because it's ill. Remember, once a bird appears ill it is already quite ill. This is a protective mechanism because sick birds are attacked in the wild.
Please respond with further questions or concerns if you wish.