'Kidney' there are thing to look for during the veg cycle too. First when you see the branching patten go from 'dipoildal' ( two branches coming from the same level of the stock, to 'monploidial', you should just be into the preflowering stage, its where these branches attach to the main stem that preflowers will show first. Either male or female, but they will show. This is supposing that they are on the 12/12 cycle too. 'Gagrown' has a good point in that for instance the stain of the plant can have a direct effect on how much light stress/change it can take before it induces it to flower and or hermenize.