You can always tell sex from preflowers unless your plant is hermie...but if you know you have normal unsexed seeds you can always tell sex from preflowers in my experience...I have been wrong 1 time on 40 plants and that was because it was a hermie. You just have to be patient...alot of times the females pistils won't come out right when the preflower shows, it may take a few days...and the males are very easy to spot once you get the hang of it...most strains have a tiny claw...at least that is what it resembles...almost similar to the way new leaves look on the plant before they open and spread out but much smaller. Usually the preflowers appear on the 4th-6th internode, and sometimes on more than one internode. Anyway have fun