You get to learn how to vent sex them, or find someone who can do it for you
Some people swear by the voice, but I just can't pick out consistant differences in voice with my geese (of which there are dozens
Vent sexing is very easy. If you know how to sex a rabbit then you're already 90% of the way there. I've found that the easiest way to sex them is to have the goose belly up and staring at your backside while you hold him between your legs. That gives you more leverage and keeps the goose from flailing and injuring himself. Bend his tail back and locate the vent, then massage and push/stretch the vent open. After some massaging you will know 100% if you have a boy since a long white, prickly spiral will unfold. Younger geese will have a small red worm looking curl that comes out.
You'll know you're on the right track if your goose poops on you and you have to try a second time. After the gift it's easier to get the genitals to show.
There are pictures all around the internet for vent sexing geese. A lot of them warn you that a female goose has a small curl that looks like under-developed genitalia. Once you've seen a male you'll easily be able to see the difference.
The other way to figure out who is male and female is to put out a trough of water this spring and sit out in the yard with a book. When you catch the lovers in the act just tag the one on top as a male and the one on bottom as a female
PS: At times you'll come across a bonded pair of male geese - even if there are other females around. You'll know your duo is an all-male team if you catch the "female" switching roles and ending up on top. It's rare but it happens.