I do not trust myself to sex a young plant in vegetation. Those pre-flowers have fooled me more than once!
I have not had a non-auto plant flower in 24/0 or 18/6. ( not including pre-flowers.)
With non-female (reg seed) when grown to desired height to begin flowering there will be male and females. What a waste (in power, nutes, space and time) growing unwanted males for so long. (And they are often the nicest biggest plants.)
Instead I choose to waste only my time by:
Veging until around five nodes (about a foot tall.)
Flowering, with a very watchful eye, until sex shows. (Getting rid of unwanted males as they show their sex.)
Reveging until desired height.
Re-flowering until harvesting.
(Caution: this may on a rare (for me very rare) occasion cause hermaphrodites.