You should be able to tell by taking some samples off a seeding bud and check. If they are pale, lime green, ... they are not ready. You are looking for the healthy, somewhat dark, veiny seeds that you would expect to see from a seedbank. You can search for pics of healthy seeds or zoom-ins of people purchases from seed banks and get examples of what exactly to aim for. Rather than the bud finishing in 8-10 weeks of flowering, expect your seeds to be ready after 8-10 weeks. And don't forget that while your seeded buds won't be the greatest smoke, you can make A LOT of hash with that after you extract seeds.
Peace!