you can tell the female from the males at about a few months of age when they get their telltale "sex feathers" in the back by their end of the rump you will see curled up feathers or not. Curled = Male Straight = Female..... as far as telling them apart i wouldnt tie anything around their legs , because there is a big risk of injury and they grow so fast that it may get way too tight too quick and you will be unable to get scissors in there to cut it . if you absolutely needed to tell everyone apart ( ie- ones not gaining weight , for feeding purposes etc) then i would use a DOT of water based paint on top of their head. Im sure spray painting them with oil based wouldnt be safe and application with aerosol could pose respiratory concerns for the little duckling.. and also they cannot go swimming for at least a week, until their oil glands fully develop to make them waterproof . with their mother in the wild they can go swimming within a day because they are coated from touching and being underneath momma. When we first got our ducks we had no clue so we went to the public library and checked out every book about ducks lol and we were very glad we did. I fed the ducks at the park bread when i was a kid , so if we didnt read up on pet duck care , we would have made them very ill .
Good luck! they are alot of fun! what kind are you getting?