interesting discussion, just my two tarnished pennies. you can sex some plants as early as 3-4 weeks, i was able to on my first grow of ak-47s. i just picked up my bible and thumbed through it and found a section that does in fact say you can sex some strains as early as 3 weeks. jorge cervantes i believe stated in one of his videos i saw on youtube you can sex them usually between 4th set and 6th set of branches after about a month. if you are a non believer, ed rosenthals(the bible i mentioned above) book p. 329 does say around a few weeks old and a foot tall. also, the more rapidly and faster growing plants are males, generally speaking, if you are really concerned about sexing your plants, you can root a cutting and flower it, hopefully in a different area, to be safe.
if you need pictures of said page in said book, ill be more than happy to take a picture for you
good luck !