Two ways:
1. Flower (12/12) the plant until it shows sex, then put it back in veg (18/6 or 24/0). It will take a couple of weeks for your mother to go back to veg and this causes stress.
2. Keep your plants in veg until they are tall enough to take clones. Root the clones and put them in flowering, no veg time. Once the clones show sex then you know what you have. This is a bit longer but your clones, bud and plants will take less stress from this process. good luck hope this helps.