Chickens I can tell hen from roo, ducks I can tell drakes from hens, but I have no clue of how to tell gander from hen.
With geese, what do you look for to know male from female?
After breeding age, the ganders are pretty easy to pick out -- they are always slightly closer to you, and carry their heads more upright. Even during the non breeding season, they will tend to protect the girls. My ganders are more vocal, and have higher pitched voices than the female geese. My female geese are more reserved and less likely to come right up to us - the males were more friendly during the non breeding season. Most of my ganders are more massive than the female geese as well, although the smallest gander is the same size or smaller than his mate, so it isn't always so.
You will know for sure during the first breeding season but until then it is fun to guess . . . . they were hard to distinguish as goslings and young adults. We had guesses about gender based on behavior and size . . . the smallest gander and the biggest goose were puzzling, but we went with behavior in that case and it ended up correct.