Vent sexing is the only 100% certain way of doing geese, with the exceptions of 2 autosexing breeds which are the Pilgrims & the West of England's & you can tell those by colour but they have to be pure & not crosses.
There are ways of potentially identifying geese & ganders without vent sexing such as size difference, the calls they make, behaviour ( I saw someone mentioned ganders lead the way more - more dominant.) Some say neck size but I've never gone by this. I'll dismiss most of these reasons based on the fact that I've bred geese for nearly 18 years & with the reasons below:
Size difference - Size can be a obvious thing to look for & often males are bigger, I have come across females quite a bit larger than my breeding ganders on the rare occasion, or the same size. I recently pulled out a female from this years hatch that I believed to be a male to be part of a breeding pair - but when vent sexed she & the other 3 I thought were females were all female.
Vocalisation - Females normally do sound "Flatter or more low pitched in sound" - Ganders are more high pitched, its very similar to ducks & drakes if you listen the females are more low pitched than the males. But in geese its hard to tell until around 8-10 months as young geese often sound high pitched & don't get their proper voices until more mature - but I have also found females that call like males on the odd occasion.
Behaviour - Ganders are normally the leaders of the pack & more aggressive & protective, but some females are just as bad.
I've only recently started getting a friend to vent sex my birds for me - I can do ducks at day old but geese are very hard & you have to be careful not to damage the sexual organs or the pelvis, so it's best to get someone who knows how to do it. If your not fussed on sexing as your birds are meat ones then don't, or if your not going to eat them wait until feb - march & see who lays an egg. If you can try & pair them up then you might see the mating ritual of them "mirroring" each other - but that's mainly in the water & they copy each other & the females will nest build. It can be hard to see if they are mating as one pair I had were really fertile - but in about 10 years I only saw them breed 2-3 times. But the male I have now prefers to show off
Note in some ornamental breeds like Africans - The dewlaps & knobs on the heads are a lot larger in males.