Download the DU app.. they have a waterfowl ID tool you can look at anywhere without internet.
I've also spent a lot of time googling pictures and videos of different ducks to get a better idea of what they look like in flight and on the water.
The best way to learn is experience. Take a good pair of binoculars to the duck blind and make sure you know what you're shooting at! You'll learn soon enough.
Hayraker's answers are all correct.
This. Not putting you down or anything but my 5 year old used this app for a kindergarten project and blew the whole school away. Kid can identify 20 species of ducks...drake or hen. He get's mixed up occasionally on the tough ones but he used this app for 90% of his education. Pics, sounds and details of the species. Great tool. It also has a legal shooting time (LST) that's based on GPS, so you're good on that as well.