1 good way is their combs and waddles will get bright red. Another way I read about is if you can get 2 fingers into their vent they are getting ready to lay eggs. Let me know how that works for ya.
I have 4 Leghorns...they will be 21 weeks old tomorrow and I just got my very first white egg yesterday. It was big compared to my brown eggs!
I think it was Layla...no more white eggs today, so I have 3 more that haven't layed anything yet. Their combs and wattles have been growing and turning red since they were 17 weeks old. They also squat down now when I pet them on their back.