Are you meaning just making sure your seed is feminized from the get-go? Or is there a way to determine sex before the flip?
"
Chemical leaf testing is getting less expensive every day and can be used on cannabis seedlings with just a few sets of leaves to test for
sex and future
potency.
These tests only require a tiny amount of plant tissue (for example a small punch-out from a leaf, or a single cotyledon leaf), so it won’t hurt or slow down your seedlings to take a test sample!
In general, the tests are available for seedlings as young as 1-3 weeks. Sex testing uses a real-time polymerase chain reaction (qPCR) test, and potency tests use Gas Chromatography with a Flame Ionization Detector (GC/FID) or High-Pressure Liquid Chromatography with a Diode Array Detector (HPLC) for testing.
Although testing can be done as early as week 1 from germination, waiting until week 3 to conduct testing on seedlings can increase accuracy, and some companies won’t conduct testing until week 3.
There are many reasons growers would like to know plant sex as early as possible, as well as be able to estimate the overall THC/CBD ratios of future buds!"
This was pulled off a website I'm sure many have come across in looking up how to grow cannabis on a single search
I'm sure there's better more accurate info regarding this.
I've never done it or really cared to delve much deeper than knowing it can be done.
I'm sure someone on these forums could speak to it waaaay better than I could