hi, those plants look good, in my experience short bushy plants have been female and taller skinny plants were males, this was true with my grow, i just offed my 2 males leaving me with one female. she was the best looking one, short bushy and now shes rockin.the males were tall and thin. by the way. if you just flower one of the plants that will just tell you the sex of that plant. either flower them all or as someone said before clone them and flower the clones. remember which clones are from which plants.
i went another route. i flowered my plant early and took clones from it 3 days into 12 12 when i saw the branches i wanted to axe get big enough. the clones are under a daylight cfl and 24-0 lights to revert to veg. then i grow them and bam they are female

or at least thats the plan. and now would be a good time to get some more lights. if i read right you only have 4 4ft tubes. maybe i read it wrong but if thats all you got you need more for all those buggers.
Ery