Well they can be sneaky in their mating but I go by my ganders shrill mating call after it's done. I can tell right away when mating takes place. But to know which females he is mating with you'd have to be standing right there watching. Only way to know when she lays is to crack open the egg and look for the bulls eye that will tell you who is fertile and who isn't. And if you want to know who is laying what egg. You can use some food coloring and put it around the vent of your females they will have a colored egg the next time they lay. Just use a different color for each one. Works for finding out what duck is laying so probably work on geese too. That's great the gander has stopped picking on the female. This is a big change for them too being closed up so time and a larger area should help a lot. Good for you getting them set up. Let us know how it goes and when quarantine is over. Ganders can also just choose one mate and have nothing to do with the others. Usually 2 he'll mate but more than that sometimes just get ignored or run away from the flock. My gander never had more than 2 females and he mated them both.