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Size
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The single easiest determinate of clown fish sex is size. Females are always bigger than the males. Keep in mind that all clown fish must be sexually mature, or at least a year old, for them to go from a sexless hermaphrodite to either male or female.
Behavior
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Since females start out as the most dominant male, they are more aggressive, and as a byproduct, they are more territorial. For the most part, a female clown may be kept with multiple clowns. These would consist of a male that has shown to be dominant over the other clowns that stay sexually immature. However, if another female is added to the group she will be harassed--often to death.
The Dance
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Clownfish pairs perform a mating dance. As you might suspect, the females movement around the male signifies dominance, while the male falls into a submissive dance.
The Brood
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Females lay the eggs. The males fertilize the eggs.
The Environment
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Solitary clown fish are always female. If a pair of clown fish exists in an aquarium, there will always be a male and a female, because if the male dies, the surviving male morphs into a female.
Male clowns can morph into females, but female clowns can never morph back into males. Conversely, neither can morph back to a sexless, or immature state.
Coloring
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Although rare, and only in certain sub-species, female clowns may deepen their coloring when they are getting ready to lay eggs.
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Female Vs. Male Clown Fish
Clown fish are natural hermaphrodites that will "sex" depending on their environment. Whether paired or in groups, the most dominant male will become female. This change to female is permanent, so while male clowns can morph into females, they don't morph back.