I'm new to duck hunting and would like a guidebook or website or something that would show me very clearly the differences between ducks. Gadwall vs. hen Mallard in flight, that sort of thing. Most will give you a profile shot, maybe mention underwing coloration if you're lucky. I think I'll be able to tell the big ones appart for the most part, but as for the small ducks I don't even know all their names yet!
I'm not really comfortable shooting first and asking to see ID later. Approaching sundown last spring in SD I almost shot a Specklebelly I mistook for an immature Blue. Thankfully I chose the Snow nearby instead as I was sure of its identity. Learned later, studying the pictures in the hunting manual, I had been right to hesitate!