Subtle things are usually seen when illness is present or approaching, and if the bird is extremely ill it will sit on bottom with feathers fluffed up.
If you notice abnormal droppings (color, consistency, size, and odor) [from what they are like usually, though they will vary throughout the day and depending on what was eaten], discharge from the nares, lethargy, depression, wheezing, coughing, sneezing [more than usual and with discharge] feces stuck to the rump area, staying fluffed more than normal, among various ither signs. If you notice something abnormal the best thing to do is call the vet and have them asess whether or not the bird needs to be seen and how quickly that should be.
Its usually just that sense that something isnt quite right. Always act sooner rather than later. With birds, if you catch something early the chances are good, if you wait until the bird is fluffed and lethargic that is usually the beginning of the end. If something is the slightest bit "off", call the vet!
Whoops, Scott got it first, and better said!
