It's difficult to tell whether your cockatiel is male or female until his first molt, around 6 months of age, unless an avian veterinarian performs a DNA test or examines the bird's pelvic bones. After the first molt, there are several indicators of your bird's sex.
Boys Are Brighter
Until the first molt, males and females look alike. Both have dull, solid plumage. One the molt is complete, the male bird has much brighter markings than the female. A male grey cockatiel has a bright yellow face with orange patches on his cheeks. A female bird's colors are duller, and her face will have pale, washed-out yellow coloring and no orange patches. Female birds may have all grey or brown faces, with no yellow markings.
Male cockatiels exhibit different behavior patterns than females. He will be more showy, fanning his wings and strutting. Male birds are much more vocal than females. They whistle, chirp and call. Put a mirror in the cage and a male bird will perform for it, chirping and calling. A female bird tends to be quiet, less friendly, and is more prone to hissing and biting.