Um.. when they lay an egg - then you can be certain. Girls can crow.....
Actually I have one 7 week old I now know is a boy - all the others I am still guessing. SQ silkies you won't know until about 5 months or more - the boys will get streamers out of the back of their heads.. and possibly start crowing. The girls (usually) won't. Some have a difference in their head crests - the boys sweep back and the girls tilt forward. Some have difference in their beaks - really curved = boy, straighter = girl. All these methods are guaranteed to be 50% accurate...
Some people have become adept at spotting what is girl type and boy type - I am not one of those but I do guess with mine. I have two I go back and forth between boy-girl-boy-girl.. I guess I will know in 4 more months!