I band mine and make notes along the way. Over time you will get better at it and know sooner. I have never had to wait until one crows but sometimes I wait to hear it crow or not to confirm my suspisions.
I used to say the same thing... but I've got a three month old splash chick crowing in the back... I think I heard him this morning, but I'm sitting outside the room listening to see if I heard what I think I heard.
Althought a crowing 3 month old is a HUGE exception to the rule. On average, you wont be able to sex a Silkie until 5 to 6 months, sometimes maybe even longer.