Roos will, in general have larger, chunkier looking legs. Larger, more quickly developed combs and wattles. The feathers on the "saddle"..the area on the back just in front of the tail, will grow long and pointy...looking rather fine..and curling slightly down. The hens will have round shorter feathers.
Roos also have longer tail feathers.The hens will carry the tail upright, and kind of LOOK male that way, but the feathers will be shorter and round..looking more like a fan.
Most breeds start developing the traits at around 3 months. By 4 months they'll crow and then you know for sure.
If these are Cornish rocks (Cornish Cross, Cornish X)...which is what is usually referred to as "meat bird", they're bred to grow fast and die early. Butcher size by about 7 weeks. If they're Rocks (a normal breed of bird, but nice and chunky and meaty), they will grow to a nice table size by about 4 months.
I've raised Cornish Crosses with other chickens free range and they live quite long without problems because they get exercise. After a year or so, tho, they will succumb to heat problems
NICE PEN, btw
I need to get a couple more built so we can separate the breeds.