Silkies are the easiest for me. Longer toes, often longer legs. Often grow out wing and tails faster than females. Identifying the sex by vent is the surest way. It's how hatcheries do it.Except for silkies
I have guessed with silkies as early as week one and been right based on beak shape. It's not a science though, and there is always room for error. I've had boys really difficult to sex (silkies).
Turkeys don't have penises though. They are just like chickens in that regard.
Oopps about turkeys. I meant geese.
I use these charts. No infringement of copyright is intended. I post these for educational reasons only. I can post images of ducks and geese as well if anyone is interested in studying them.

I just now went to this one & the Texas thread & everyone is posting away
but I don't know about it....got my 1st Wyandotte egg yesterday from one of my 2 girls 8 1/2 months old. The picture is a WR egg, an American Game & the little 1st Dotte egg
and 
