Well once your plant has about(give or take 1) 8 nodes you should start to see preflowers just hope you dont start seeing little clusters of balls.. If you dont then you should be in business
But sometimes you could tell which ones male or female by the way they look, males stems tend to be longer and thee plant more diffuse in general, and fem are more tight and full looking(what I mean is you cant really see the stems of a female if your looking at at it from far away).. guess which one is fem in tha pic below??