Male ducks in some species are easy to identify:
This becomes trickier with females. Take the Pekin Duck for example.
If the duck has a curled feather at the end of the tail, it is a male.
No curled feather means female.
In addition, female Pekins have a loud honk and the males give a quieter quack.
Female and male American Black Ducks also look alike. The difference is that the male has a yellow bill.
Here are photos I have taken of 21 species of Ontario female ducks.
LESSER SCAUP (the differences between Lesser and Greater Scaup are subtle)
She was a Phantom of delight
When first she gleamed upon my sight;
A lovely Apparition – William Wordsworth