Here is a mature preflower of one of my plants , it is the thing with the two white hairs comming out of it in the center of the picture. Taken a couple weeks ago.

Female and Male preflowers both start out looking similar, and they form where the branches split at , the first ones usually appear near the middle of the plants at first. Female preflowers kinda resemble a teardrop and eventually will grow 1-2 hairs comming out of them. While male preflowers resemble a "ball on a stick" and will grow in patches of more then one. Dont ever kill a plant until you see the telltell signs of male preflowers growing in groups of 2 or more next to each other. As a plant gets more mature when it is in veg for a long time a plant will show lots of preflowers and are very easy to determine sex by. The first preflowers start usually forming when a plant reaches maturity.
(when the plants nodes start alternating instead of forming in pairs it has reached maturity usually takes around 2 months from seed for a plant to reach sexual maturity)