The earlier you try to identify the sex of a cannabis plant, the less accurate your guess is going to be. There are a few early signs, but they aren't really reliable.
If, however, you're working with regular seeds or are unsure of your seed type, knowing how to determine the sex of your plant is vital to.
Cannabis Pre-Flowers: Identify Sex as Early as 3 Weeks Old (with pics!) . Unfortunately, you can't tell a cannabis plant's gender for sure by.
As we all know there are many uses for hemp. That should be reason enough right there to keep your male plants.