you can do it, but it does take time. On average (imo of course) it takes around 2 weeks for a plant to switch phases, so potentially you could lose a month (this is all strain and environment dependent of course). A number of people sex this way though. Two other things to consider are to cover a small branch with a paper bag for 12 hours a day, tie it to get it light tight (black paper bags doubled up work well), within 2 weeks you usually can see sex. Another thing you can do is cut a clone, and set it on 12/12, track which clone came from what plant, and see what sex the clones show. Alternatively you could cut a clone, veg it, and flower the parent...