Ganders have larger necks and look more like "males." They also have a different voice.
Just so you know, this is just as reliable as assuming that men never have pencil thin necks and that there aren't women out there built like a solid tank. And that men never have high pitched voices and women never have a deeper sound. Sure you may guess right often, but "often" isn't accurate.
I've found that the safest way to sex them to be 100% sure is by vent sexing. It can be done, but it's tricky if you do it when they are no longer goslings. Typically between 2-8 weeks is the best time to figure out what you have.
Here are a few things I have found that has made sexing easier.
1. Hold the bird so their belly is facing your belly. If you are dealing with an adult, hold them firmly, but gently, between your legs - their belly facing you and their head looking down at the ground.
2. Find the vent (easier said than done on fluffy adult geese
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3. Bend the tail back away from you. If you are dealing with an adult, you should have the bird braced using your legs, so you are free to manipulate the tail and vent with both hands. This is important.
4. With the tail bent back using a few fingers, take your thumbs and put them on either side of the vent. You are trying to fold it out - for lack of a better way to put it.
5. If you are dealing with a _maure_ adult male (at least a year old, preferably older), then the genitalia should have little white spines all over it in a spiral shape. If you are manipulating the vent and see these spines poking out, but the genitalia itself doesn't pop out, then you aren't doing it right - but you are doing it right enough to know that you probably have a male. With a little more wiggling from one side to the other you can expose it fully.
6. If you have a female, it will be more of a caved in hole when you are peering around inside. Males tend to "bulge" out. This is not 100% - until you see the genitalia you could be dealing with a well-hidden boy.
As a baby, the genitalia is easier to expose and will look more like a fishing/trout/compost worm without any spines.
The indignity of being held upside down and prodded is not something any goose likes. But if you do it right you only have to do it once. After that band them, release them, and give them treats to help them forgive you. As babies it isn't a big deal. They shake it off and get over it pretty quickly. Adults will hold a grudge for a while.
I hope that helps.
