Birds have evolved to hide illness where at all possible. It is therefore important to look for subtle signs of ill-health in your pets, as well as the more obvious indicators. The Association of Avian Veterinarians has produced the following leaflet which lists some of the things to be aware of, and which may suggest your bird may be in poor health, or heading in that direction.
We also strongly recommend that your take your bird to see a specialist avian vet periodically. They may be able to identify potential problems before they become too severe, and run any necessary tests.
If you are concerned that your bird may be unwell, don't hesitate in contacting a specialist avian vet and arranging an appointment.