One method of sexing Africans is by looking at their knobs on their beaks. A gander's knob will be larger than that of a goose. However, I don't raise geese myself, so I can't tell you exactly what a "larger" knob looks like--I bet it is hard to tell unless you have both genders next to eachother.
For more answers, I would definitely post this in the waterfowl section of these forums.
Feel free to ask any other questions that you have! We're all here to help.