I have currently have 14 Sebastopol geese, and I always had hard time telling which ones were goose and which one was gander until they start laying , As I was searching the internet, and asking some people in a group I am a member on Facebook they recommended Health Gene . You can either send in feathers or blood samples. I found blood samples to be easiest for me. You go to their website and request a sample package to send in and they will send you a form and envelopes with cards inside to fill out and send in either the feathers or blood. I highly recommend this place for all you DNA sex needs after my geese are done, I am going to move to my guinea birds. It does cost $15.00 a sample but is worth it if you want to get your breeding pairs together before spring or for the following year.