It can be as simple as a strain to more important indications of trauma. You also want to pull a few feathers back and look at the skin. A yellow cast is an indication of excess fat. If the bird is on the fat side you will want to make some changes to the diet. Check the feathers.
Feathers can reflect diet or environment.
A healthy bird will have shiny feathers and be free of dirt or a dusty looking appearance. The feather shafts will be normal, not thin or weak looking. Disregard broken tail feathers in young birds as many macaws will not learn to appreciate that long thing that gets in its way. It is seen as a nuisance, thus it's normal for a young bird with a long tail to break its baby feathers. It may take a couple of years before it becomes a thing of pride.