In my experience it is quite difficult for beginners to determine what is a true male/female pre-flower. (Even many experienced growers have difficulty at times.) Not to say that you couldn't do it SeaMaiden, 'cause how the hell would I know?
I would just hate to see you pull females by mistake. I never try to judge sex until bloom but there are many that do. If I really need to know the sex of a seedling I will take a clone and put the clone in bloom. For me, that's the definitive method. Here's some info for you from my bag o' knowledge ...
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"Identifying a true preflower is way to tell sex before changeing to 12/12. That way you can take clones from the known females without wasting the time and space on males.
(Kifit) "Do not try to sex a seedling based on the very first preflower you see with a 25x times microscope.....wait and make sure. The time between using a 25x to spot the very first preflower sex and the plant dropping pollen is at least 10+ days away and so it's safe."
"It's best to cull a male only after you are 101% sure - when you see 2 or 3 (or more) immature male flowers bunched together on the internodes or the top growing tip - this is a male, for sure, females preflowers have white " spears " that appear in a vee. ..but "every now and then a sexually indistinguishable flower appears" (Ed Rosenthal)
After a few weeks in veg, plants will begin to show their sex. Usually the males show first. The male preflower is a miniscule ball. It appears that there is a small piece of foliage that covers the ball and protrudes outward when the male preflowers first appear.
Many times preflowers will appear at the fourth or fifth node, whereas the plant is on the 7th or 8th node. These preflowers usually don't develop into full flowers, but are only an indicator of the plants sex.
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More ...
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Most indica strains will preflower anywhere from 4-8 weeks after the first set of true leaves develop, but that doesn't mean all strains will. Some sativa's won't even show preflowers until a few weeks into flowering. When preflowers first start to develop they are unrecognizable as to which sex they are. They'll need a few more days or up to a week once the first sign of them to tell which sex they are, so don't jump the gun, wait until you are 100% sure of the sex before you toss your males.
Sexing a plant can be done by inducing flowering by cutting lighting down to 12/12 or by just letting a plant mature and preflower by itself. Once a plant starts alternating branches the plant is mature enough to fully flower with the best results (for yield and quality). Alternating branches usually starts on a small side branch first and will move to the main stock some time after that. all you need to see is the first set of alternating branches and you know the plants are ready to flower, even if the plant is not fully showing preflowers yet - but most will show at this time.
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Just some thoughts ... hope it helps.