1 dog in each litter will stand out from rest on their NOSE.
it could be male or female.
a dog trainer can pick that dog out in minute.

NOSE is what you look for,if puppy looks like its nose is stuck to floor,good chance that will be one.

i saw about 8 litters of american labs pups.
in about everyone,1 or 2 pups would fly around room with nose going,SNIFF,SNIFF, SNIFF,i mean really moving with nose stuck to floor.

others would walk around, sniff, look,run with head up.

this is not perfect way to pick pup for hunting as each pup has bad day, tired etc ..

BUT if you can get a look at pups at 6 weeks,then again at 7 weeks,you can get idea of one that uses nose the most..

our breeders put little tag on ears of pups at 6 weeks so when you come back at 7 weeks, you know which one you looked at.

my 2 cents on this only........