Plants in constant light can sometimes start to show some pre-flowers after only a few weeks of growth. Usually though it takes a change in light cycle to trigger the plants into full reproductive mode.
Most varieties of marijuana require 12 hours of darkness per day to induce their flowering cycles. Sativa varieties sometimes need more. Indica varieties can sometimes take less. Ruderalis varieties can flower whatever.
Early female marijuana flowers look like little furry hairs, usually white, but they can be other colors. Early male flowers look a bit like small bunches of mini green bananas. Look near the leaf joins on your plants to see them emerging.
Even pure female plants have the potential to turn male or hermaphrodite. This can happen if the plant is stressed too much through bad treatment or if chemicals are applied to the plant to induce male flowering.