When can you tell the gender of photos ?

Before the 12/12 switch it's hard to tell , the plant can show her sex but not always. If you wanna know the sex before the flowering ( for a photoperiodic strain ) then use a paper towel tube , and put that tube onto a stem , put a dark tape on the top to block the light and wait for a week to 10 daus , the part that will not receive light will get into flowering and show sex while the rest of the plant will stay in veg