It is best to get a good scale (e.g. a bird scale, postal scale, or any other scale that weighs in grams) and periodically monitor the weight of your bird. Birds are experts at hiding signs of illness, so noticing weight loss is often the best way to detect a potential health problem early. Experts recommend weighing baby parrots daily, and older parrots should be weighed at least once or twice a week. Keep good records of your bird's weight so you can notice trends quickly. A 5 percent loss of weight is very serious for your bird, and will only be detected by regularly weighing your bird.
Feeling the Keel Bone
This is very imprecise and definitely not the best way to monitor your bird's condition over time. However, when you are picking out a new bird it is a good, quick way to assess body condition and avoid underweight (and possibly sick) birds. The keel is a long, thin, flat bone that protrudes at right angles from the chest wall (breast bone) of the bird. Muscles attach to either side of the keel bone, and the edge of the bone can usually just be felt running down the midline of the bird from the chest to the belly. To feel the keel, hold the bird on its back and feel for the keel on the midline of the chest and belly with a couple of fingers. The keel runs lenghtwise down the chest and belly, and it is best to feel for the prominence of the keel by gently moving your fingers side to side over the keel.
Normally, you can feel the edge of the bone but it is more or less even with the muscles on the bird's chest, so it is not very prominent. However, in a skinny (underweight) bird, the keel bone is very prominent and the edge of the bone feels very sharp. In an obese bird it is very difficult to feel the keel (often there is just a groove where the keel would normally be felt). Newly weaned birds are often a little on the skinny side, but ideally you want a bird that is in good condition -- where you can feel the keel easily but it is not overly prominent.