Well, there's a problem with putting your plant into flowering to determine sex, then moving back to vegetative: you will lose about a month of grow time. It will take about a week or so to show sex in flowering and then about another 2-3 weeks to turn back when you take it back into veg.

You should consider cloning to sex your plants. Root some cuttings and put them into flower; if the clones sex female, so will your mother plant.