Well the only 100% way to tell the difference is that they crow or lay an egg . But there are other pretty accurate ways to tell like if their comb is already really big and red and they are still pretty young.Another way is to look at your pullets saddle feathers.Most pullets don’t really have long saddle feathers.Saddle feathers only start to show up when the chicks are around 14 weeks though.And to sex a silkie (incase you have some and want to sex them too) there is a pretty accurate way and that is too see if they have streamers coming out of the back of their crest.If they do then again you’ve got a Roo.Good Luck! I hop I could help you!