I can advocate the metzer link. I used this to learn how to vent sex poultry, but do prefer to do it when they are small! We recently took on 6 extra geese, and they are very different (same breed) in their vocalising/behaviours. My original group, the females have the deep butch voices and tend to chunter to themselves. The boys have that high pitched squeakier version. The boys tend to front up to me if I am moving around...not attacking, just keeping between me and the girls. Also with my geese I noticed that the boys have a flatter head top, while the girls heads are more rounded. Based on this, I do think my american buff is a boy not a girl...and has paired up with my orphan hatchling of the same age, Solo but so far we think we have a roughly even number of boys/girls.