This sounds like it needs to be in the "What Breed or Gender" section.
The cockerels will grow alot slower than the hens, they will also have large legs, where as the pullets will have small, slender legs. The cockerels will also develope a big comb and wattles. You will also start to noticed shiney feathers all over their body, and their tails are long, shiney, and flowing.
Once they're fully feathered and no longer need their heat lamp, they can join the flock.