Sexing mature geese is very easy buy vent sexing them. Hold the goose upside-down in you lap with its tail facing away from you "put the head under you legs to prevent biting your arms". Gently press around the vent with both hands. Fingers on top and thumbs bottom, so that you are pressing all the way around the vent. You may have to manipulate the vent a little as well. If its a male the "reproductive organ" will pop out, looks like a furry corkscrew. If nothing, its a girl.