How can i tell if my ducks are male or female.?

I got three WHITE PEKIN ducks the monday before easter there not full grown but not little ducklings either. How can i tell if they are male or female. Its O.K. if you can only tell wether there male or female when their older still give me an answer.
Answers:
Its very difficult to tell gender when they are young. When they get older the male will get a curled tail feather. That's really the only distingushing characteristic between male and female.

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look for the penis

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i now i now ! look at there noses the darker one is a gril and the lighter one is a boy i have 2 brids (i am going to be avet when i grow up)

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The males are usually more colorful. And have curled tail feathers.