There are Youtube videos on this which have been included in previous posts but it's something you do at day old or thereby otherwise it can be dangerous, you can hurt them. Not sure about doing it when they're older - can't you tell by the size of the head, thickness of the neck or legs or behaviour? We have Steinbachers so not quite the same as yours, the difference in ours is quite subtle but it's easy to tell at this time of year. There are quite a few people on here who breed geese, if they don't come on in a few days maybe repost, good luck.