Instructions
Listen to the cockatiel. While it is a false assumption that female cockatiels cannot sing, they do sing less than the males. If the cockatiel is very vocal, it is likely a male.
Look at the cockatiel's face. Typically, male cockatiels have brighter faces than female cockatiels. This only applies to varieties of cockatiels where there is a distinct facial color. This method cannot be used on albino cockatiels or whiteface cockatiels
Observe the cockatiel's behavior. Males tend to strut and display, where they lift their wings, puff out their chests and call out. Females are less prone to this behavior.
Look at the cockatiel's tail. While males have a solid tail, females have faint stripes on the underside of their tail feathers. This difference is very subtle in paler birds and it is not distinguishable at all until the bird has molted.
How to Tell if It's a Boy or Girl Cockatiel
Sexing cockatiels is an essential part of breeding them. The only way to be completely certain whether your cockatiel is a male or a female is to have their blood or eggshell tested by a laboratory. A trained avian veterinarian can probe your cockatiel, but this process requires extensive training and care. While males and females are visually identical until their first molt, afterward, there are methods that give you some hints as to the cockatiel's gender.