Breeders do not go to all that effort to make feminized seeds for nothing. Even then this not 100% reliable you still get the odd male or hermaphrodite plants if put under stress while growing.
While being able to determine the sex of a cannabis seed by looking at it is a myth, being able to sex your cannabis early is not.
Normally, you would wait until your cannabis plants are done with vegetative growth, and switch them over to flowering. You then watch like a hawk for any signs of sex, and move the males as soon as possible. This is risky, though, because if you leave it too long, you risk the chance of pollination – turning your bud into seeds. Unless you are getting into breeding yourself, this is something you probably want to avoid. Myself this is the way i do it when growing a regular seed with feminized seeds.
If i am growing just regs then i want to sex them early how do i do this?
Well there is several ways to do this, one of my favourite ways is to veg the plants for 2-3 weeks then flip them to 12/12 and wait for them show sex.
You will need a loupe and closely monitor them, Once you have determined sex you weed out the males and then reveg your plants for a couple of weeks, a little long winded but works a treat.
Another way is to use a black bag over a small section of the plant during the vegetative stage to force pre-flowering, before allowing it to revert back.
Place a light tight, black, breathable bag on a lower branch of each plant for 12 hours a day.
It works by stimulating pre-flowering in that part of the plant; and as soon as you begin to see its sex, you can stop placing the bag over it, letting it revert back into its vegetative growth.
Do not be scared to grow regular seeds as it will give more choice when picking your seeds. Also some growers claim that the weed is more potent from regs seeds (personally i am on the fence with that one) but you can get some cracking strains that only come in regular seeds.
With that in mind i will leave you with a couple pics of a male and female parts.
Male plant
Female and male comparison