I can see the tops of 2 of the plants in the first picture and it does not appear that there are alternating nodes yet. You can see in your pics that the branches are growing out directly opposite one another. When you get alternating nodes, you get one branch growing out and them a little ways up the stem, another branch is growing out at a different level and so on--they are not directly opposite one another any more. This indicates sexual maturity and you can then start looking for preflowers.
Height really has nothing to do with whether they are old enough to sex or not. The plants are quite stretched and don't look like they have many internodes. How old are the plants? Have they been outdoors their whole lives? If indoors under artificial light, what kind and size?