yeah, you can tell the sex in vegetative stage but only with some experience. if not sure, watch very closely. re-flowering is very stressful, but as knowm said, can be very productive. also, try to keep temperature as constant as possible - that is, same temp each day and same temp each night. remember, plants grow from light AND heat. with proper water and co2 plants can take heat as high as 117*! good luck, rock on!