Ducks make a horrible loud sound people call a "quack". I don't understand why they call that sound a quack ??

Drakes make a raspy sound.

Drakes have a curled feather in their tail. Ducks don't.

 

I have black sweedish ducks and they (males) surely do have that curled tail feather

 

Voice is the surest way to tell. A drake may or may not have a drake feather grown in, but your black swede boys will also be very likely to have green heads or green tinted bills! Females will probably have black feathers on their heads.

This video has the comparisons, they show the boys voices first, then the girls'
[ame=http://www.youtube.com/watch?v=gut7RIAN094]Male and Female duck - YouTube[/ame]

 

Voice,
Tail,
Bill, in colored ducks the drakes will have a green color base to their bills, the ducks will have a brown or yellow brown bill color base, often they also have a dark saddle blotch on the top of their bill also,

post a pic of them, heads and body shots and we can tell you what you have,