Your best bet is to keep your pet cockatiel in a room that is anywhere around 65 to 80 degrees Fahrenheit.
01.
If its wings are a bit droopy, if it does not respond to you, if its feathers are ruffled, or it is exhibiting other unusual behaviors, then it is most likely sick.
02.
This will be especially difficult to notice in your cockatiel. This is because cockatiels like to keep such behaviors hidden.
03.
If you see your cockatiel do this, and then it falls asleep shortly afterward, you have nothing to be worried about.
04.