For a female look for pistils. At first, you will only see 2 white pistils at each node. This is when you sex and can still veg. As flowering begins, where nodes begin, and at the top shoots of your plants you will begin to see many white pistils form. Here are pics of female preflowers as my one indoor female just did start. Every place you see all those white pistils sticking out wildly, bud will form. As flowering continues more and more of these "bud spots" will pop up and form into our favorite flower.
For males there will be no white pistils at all. There will just be clumps of green balls forming. These are the males pollen sacs and indicate its time to chop the plant with the balls