Adult male cockatiels are brighter and more vibrant in color, with brilliantly colored heads and defined, contrasting cheek patches. Females are slightly duller, with less defined patches on their cheeks.
Immature cockatiels are harder to determine sex for because they are visually identical until they have had their first molt. Both male and female birds at that stage have female coloration.
Certain color patterns for the bird also make visual determination of sex impossible. Lutino pattern cockatiels look identical regardless of sex. Pied birds are also not sexually dimorphic in regards to coloration. Lutinos are yellow or yellow with white torsos, and pieds have patches of white all over their bodies.
The whiteface pattern lacks cheek patches entirely, but males can be readily distinguished from females as the males have a larger area of white on their face.
Palpitation of the bird's hips can help resolve confusion over the bird's sex. A female's pelvis is generally broader and more flexible to allow for the passage of eggs. A male pelvis will feel more narrow, stiff and pointy.
When all else fails, DNA testing can determine the sex of the bird. A few feathers can be taken to a vet who specializes in treating avians. Freshly plucked feathers are recommended, as molted ones do not have enough DNA.