Duck Vent Sexing
Younger ducklings can be "vent sexed", which involves their vent being examined to identify male/female birds. Birds which show as male we can be very confident on. However birds looking female can occasionally be a misidentified male. Although they are very likely accurately sexed, these won't be sold as guaranteed females.
When the birds are older, around 5 - 6 weeks, other signs are visible in most breeds. Therefore if you want to be more sure you are buying females, please purchase more grown on birds.