I vent sex my geese. Any chance you feel confident enough to try that?
Otherwise, a gander is bigger than a goose. A gander's knob is noticably larger, usually. The gander's voice is more shrill, but he can also make the lower honks. Same for the goose; her voice is lower, but she can make the shrill noises. It is a "most of the time" type of thing. Ganders also tend to stand more upright.
A description of the vocalization differences from Majestic Waterfowl:
"Males have higher-pitched voices than females. Males tend to be quite vocal and louder than females.
Meanwhile, females are not nearly as musical as males and have a lower-pitched and monotone voice. They make a sound that is often referred to as an "oink." If you pinch your nose and say "Oink" in a low voice, you'll get the idea."
With only two though, it's hard to compare, since it's very possible you could have two of the same gender.