Mine are autosexing so much easier but I'd have thought you could tell by behaviour. My ganders will come up and see me (with varying degrees of animosity depending on the season), the geese hang back a bit more. And, yes, they are slightly smaller but not sure about the pouch - when they're young, I think the size of the pouch is more a feature of age than gender. My pair from last year both have lovely big pouches, my young goose who was hatched early in the season is just getting one and the young gander who is the youngest is still quite streamlined. But like I say, mine are a different breed so maybe this is completely different (and useless to you!),
H