I don't know about sexing cochins but, silkies are probably one of the hardest breeds to sex young. Generally they are 4-5 mos. of age before you can sex them, some are even more difficult and it is not till they lay or crow that you know for certain. They are a breed I advise people to buy as older birds, point of lay pullets, because you can end up with a bunch of cockerels otherwise
You may want to ask at "Silkie thread," which is the longest running of several silkie threads. My friend has had silkies for more than 25 years and still got "fooled" by "stealth" roosters, including her most recent acquisition at 5 mos. of age. She has kept them regardless of it being prohibited in her residential setting.