Females usually have barring under their tails.
And males usually have brighter cheeks.
In the wild male cockatiels have a yellow head and orange cheek patches and no bars on the under-side of their tail feathers. Wild females are grey all over with bars on the under-side of the tail. Over years of intensive pet breeding have make it impossible to tell a male apart from a female by colour. Ten years ago you could make an educated guess of the sex of a cockatiel by looking at the underside of their tail. Back then a fully mature, adult female cockatiel would develop the distinctive bars on the underside of the tail whilst a male would not develop bars. Nowadays these bars are being 'bred out' so they are no longer an indicator. Today, behaviour is often the best clue. Female cockatiels will lower their bodies so they are horizontal and chirrup sweetly when the human male that they love best returns home. Whereas a male cockatiel will tend to stand up tall (vertical posture) and be more inclined to bond to the human female member of its family. A female cockatiel normal voice is a squawk, whereas a Male's voice is usually melodic and pleasant. So posturing to humans and voice are your best indicators for determining gender. The cockatiel on my avatar image is a female: Proof that colouring no longer indicates gender. If you really want to know if your time is male or female an avian vet will confirm it for you.
You could go with the male having bright orange cheeks and the females being muted,but the only way you know for sure is to have the bird DNA tested.
You have to look at the little orange patch on their cheeks. You need one bird where the patch is surrounded by white, and one where it is not totally surrounded by white (but I can't remember which is male and which is female!).
You can pick it up turn it upside down and look at the special breeding **Ahem** tunnel