I'm sorry I just looked at your number of posts and didn't realize that you might be totally new!
Generally speaking a roo will look longer and leaner than a hen, stand taller, have their head up more "on the look out", have thicker legs, larger comb/waddles and turn red quicker, be more aggresive try and pick more fights, establish dominance, then develop saddle and hackle feathers, and depending on the breed be more colorful, oh yeah, then of course start to crow and mount the hens. Hens will be smaller, shorter, more round body, more solid colors more plain & less flashy, and no saddle or hackle feathers, but instead have a "cape".