When they are really little babies you will not be able to tell.....ALTHOUGH....BluegrassSeramas and I have come up with this method that seems to work about 75% of the time........ You pick up the little chick gently and turn it upside-down and if the little chick hold its feet down and rests, then its a girl. If the little one kicks and kicks and won't stop or if it holds both legs up towards its head its a boy.
I don't remember where we heard this but I do it all the time, and I have been right and wrong.