Coloring

A couple of cockatiels.

The male cockatiel, right, has more
distinct colors than the female, left.

The easiest way to tell male from female cockatiels is by their coloring. Males will have brighter color in their faces with more pronounced cheek marks, while females will have more of a blended look.

Unfortunately, looking at colors to determine a cockatiel's gender has its faults. For one thing, the coloring may not fully develop until the bird is a little older. Another thing is that in some mutations, such as the Lutino or the Pied, the males and females look almost exactly the same.

Whistling patterns

Another way to determine your birds gender is by how much it whistles. Generally males will whistle more often than females, and they will pick up more tunes. Females generally do not learn to talk either, although there are always exceptions to the rule.

Habits

Rembrandt flexing.

Rembrandt, my male cockatiel,
flexes his wings before whistling.

Lastly, males have different habits than females. For one thing, males will sometimes spread their wings a bit to look like they're flexing (especially when whistling).

Males will also bang on the bars of their cages from time to time, presumably to attract females.