Calyxes and pistils at the plant’s nodes indicate that the plant is sexually mature and ready to flower. Flowering starts when you start seeing pistils at the tips of the plant.
Is the an auto or a photo plant?
Its a photo
the best way to tell when they are about to flower is when the stretch happens. right before they start producing flowers plants like to stretch out to make space for the buds. they will increase in height over night basically. keep in mind, they are not growing, meaning they are not producing new meristems. they are just like i said before stretching to make space.