These are your key indicators that something isn't right with your companion bird. Changes in their eating habits & remembering if you've given them any new foods recently. For Example; fruit can cause watery stools.
Any change in dropping color or consistency (ie watery droppings), or behavior that isn't how it normally behaves. You've adopted a living creature that's going to rely on you completely in order to stay healthy and happy.
As you & your companion bird become closer (bonding) it'll become easier to notice things out of the norm. Birds naturally hide any signs of illness because, in the wild, it would target them as easy prey. Its up to you to be observant, paying close attention for any of the following classic signs of sick or injured birds; keeping feathers fluffed-up, drainage from the ears, nose, mouth, or eyes, dull eyes, drowsiness, stops talking or singing, hunched posture, staying at the bottom of the cage, stops eating & drinking, sunken breast bone easily felt. A bird showing any of these signs is usually well past early stages of being sick and is in immediate need of emergency care from an experienced Avian Veterinarian.
At the first sign that something is wrong with your companion bird, examine it from head to toe immediately. This'll help give you an idea of whats wrong, & help to decrease additional stresses beyond what the bird is already experiencing. Your bird is going to tolerate you examining it better than if they're in a strange location, being handled by a stranger.
If you decide that something is happening with your bird, call your veterinarian's office right away, give them the details of what you've discovered during your visual exam. Then let your veterinarian decide what next to do (and do it). Warmth is a major factor in keeping the condition from worsening; Wrap the bird in a warm thick towel (you can warm a towel in the microwave or dryer quickly, be sure that its not too hot or it'll burn them).
Birds are birds, not dogs, not cats, and not humans. You can't treat them with medicine meant for people or other animals. You might be mis-reading the symptoms. Even if you happen to guess what is wrong, the medicine you give them may not react the way you hoped it would. Always be concise and stick to known facts when talking to your vet on the phone.
If you've decided to become a bird owner/companion, then your next step is getting an Avian Vet! You don't want to be looking for a qualified doctor for your bird when somethings already afoot. Birds condition can worsen so quickly that you don't have the luxury of looking around after the fact. Emergencies just about never happen during business hours Monday through Friday. Usually something is going to happen on a holiday, at night or during the weekend.
Note :: The importance of having a licensed Avian Vet cannot be over-emphasized. These vets go to school much longer for their degrees and are very dedicated to the care of birds. A regular vet just isn't going to have the knowledge needed to properly care for tropical and exotic birds.
