Cockatiels are birds belonging to the genus *Parrot* in the family Psittacidae. Sexing them typically relies on both external observational features and physiological characteristics. Here are some common methods for determining the sex of cockatiels:

Physical characteristics: Males and females of some cockatiel species differ in appearance. For example, males may have brighter colors, richer plumage, and more prominent or fuller feathers on their head and neck. In contrast, females may have relatively duller colors and flatter plumage.
Scale color: The scale color of males and females may differ slightly in some cockatiel species. For example, the scales of males may be more reddish or orange, while the scales of females may be closer to gray or blue.

Inverted grip strength: In male cockatiels, the Wilson's round shape is accompanied by an inverted membrane between the feet. Males have relatively strong grip strength because they need to maintain it during mating.
DNA sex determination: If a higher degree of certainty is required to determine the sex of a cockatiel, DNA sex determination can be performed to accurately identify the male or female. This usually requires collecting a blood sample or plucking a small amount of feathers from the bird for analysis in a testing laboratory.

It should be noted that these methods may not apply to all cockatiel species and are for reference only. If you need to accurately determine the sex of your cockatiel, it is best to consult a veterinarian or a professional bird breeder.