Ducks look alike, that's a simple fact. It is difficult for a non-duck farmer to tell the difference between a male duck, (drake) and a female (duck). But there are some subtle differences that can tell the tale without using the difficult and often dangerous option of vent gender checking.
Look at the tail feathers of the adult birds. Drakes will have what are called "male feathers" at the base of their tails that curl up. Females do not have this type of feather feature. If the drake is molting you will most likely not see the curled feathers so you can tell by his voice that he's a drake. Only the females actually quack, while the males make a hissing squawk-like noise.
Tell baby ducklings apart without vent checking. The drakes usually have bigger feet than the ducks due to the fact that the males will grow to be larger than the females. If you see a lot of ducklings all in one place, look at them very carefully and compare the sizes of their feet. Usually you will be able to see the differences quite easily.
Look at the bill. Quite often ducklings of all one breed will have different colored bills. Typically this is a good means of determining gender as the females usually have dark brown bills and the males often have either green or orange bills. This is not entirely fool-proof of course, as some ducks, the White Pekin, for example, possess orange bills in both genders. In most other breeds you will be able to determine male or female by the color of the duckling's bill. Most duck farmers prefer to get more females than males, as the drakes tend to fight if there are too many of them.
Know that some breeds of ducks have genders distinguishable by color. This means that you can tell just by looking at their colors which gender they are, because the males and females each have distinctive colors. Not all ducks are like this so you will need to do your research in order to find out which types can be identified in this simple fashion.
Be aware that when ducklings get a bit older they will start to take on similar characteristics. Until they are nearly full grown and fully feathered out, you will have difficulty telling them apart except by their voices. Since only the females quack, they will have stronger peeps than the males. Males have weaker voices to begin with, and even as soon as the ducklings hatch you can start to hear the difference in the loudness of their peeping.