How should I know that my cockatiel sick?
When a cockatiel parrot become sick, he/she may avoid to eat and drink. Possibly, they stop singing and whistling.
If your pearl cockatiel is suffering from respiratory illness, he/she will start coughing and make some breathing noises.
If the cockatiel parrot puffed his/her feathers than you might have to understand that he/she suffering from illness.