[BKKG]

Depending on the strain, you may be able to make an educated guess from preflowers 3-4 weeks into veg, but you'll only know for sure when you see the flowers beginning to form a week or so after you switch to 12/12 (though you'll have to watch the lowryder more carefully, since you won't have to switch lighting schedules for it).

As for when to transplant, that's a matter of preference. But you can transfer to a large pot whenever you want and let the plants grow there for the rest of their lives. I'd recommend doing it earlier rather than later, and I'd absolutely not recommend leaving the plants in the plastic cups until you're able to sex them.