The white one in the last pic is certainly a roo. The rusty brown color coming in on the shoulders indicates it. I think the barred bird is male. He has no tail yet. I think the white one in the second pic is a roo. Looks like his tail feathers are starting to arch downwards.
If the barred one hatched in March I'm going with pullet. A 7 month old cockerel would definitely be obvious! Also in regard to the lack of tail feathers... I'm thinking she probably is in the process of molting and the tail feathers are growing back. I had a couple hens molt their first year so it's not unheard of.
Generally when looking at pictures, I look for pointy saddle and hackle feathers (saddles come in on the back near the tail and hackles are on the neck). A male's saddles and hackles are skinny, shiny, and pointed, while a female's will be rounded. Also if the age is known, one can sometimes go by the size and color of the comb/wattles, although the saddles and hackles are the best indicators.
All are pullets except numbers 3 and 6. I'm on the fence about number 6. The rust color on the shoulders does usually indicate a roo, but it continues down into the tail and isn't totally a roo color pattern.
It would help a great deal if we knew the ages of the birds.
Also, you wanted to know what we are looking for. Some basic things we're looking at are:
Presence/absence of hackle and saddle feathers
Size of the legs and feet