Geese are tough to sex, that's for sure. The best way to tell gender (as adults) that I've found is to closely watch their behavior. Does one sound the alarm more than the other? When they nap, does one stand guard? Of course, they will also start mating soon if you have a pair, though I have sure seen my fair share of same sex mating behavior when there is not a male around.
If you have other waterfowl, that can help decipher their gender as well. I have noticed that my goose hangs out with my duck hens a lot more than my gander. If one of them is off on their own, it's him.
Sorry that I can't be of more help. Hopefully someone with tons of goose experience comes along with better information.