You can take this with a grain of salt, but I have found it to be true more often than not. When I was kid in the early 60(s) I had 2 older first cousins that grew weed religiously, and one them, my cousin Billy, said when dealing with regular seeds if you look at the third node up and find that the stipules are crossed before any preflowers are shown it is most likely a male plant, as a female will only cross her stipules after she has shown preflowers. He said he learned that from a old weed farmer out in Monterey California!