there are pics of males and females in the marihemp.com pic room in various stages of developement. a short search ought to answer your question. your pic is HUGE and i didn't wait for it to d/l. try a diff setting on your camera so it doesn't take so long to load...

start looking for preflowers lower on the plant. they develope last near the tops and usually 1st show around where the top third of the plant starts...also no sense in looking too hard unless alternate branching has already begun.

also a 20-30x magnifying glass would help immensely...