Differentiating between Roos and Hens as far as visuals will be some of these.. there are exceptions in all types. These are just tips to help.
Usually Roos are much bigger than Hens in some breeds of standard sized chickens. Not so much in the bantam smaller breeds.
Roosters will normally have bigger combs and wattles starting at an early age and then on.
Roosters are sometimes the more colorful in some breeds. Almost like nature wild birds.. examples like Cardinal and Finch males are more colorful than the duller females.
Roosters will normally have longer thinner hackle, saddle and tail feathers that are pointed where hens are rounded.
Obviously the Roosters will have their crow and dominance behavior but some hens exhibit this also on occasion. Usually more bold and daring at a young age.
Obviously these are not all true in some breeds but if you Google a certain breed and look at images you'll quickly notice/learn the differences between males and females of that particular breed.
Here's my Black Copper Marans as an easy example. The hens only have copper in their cape or hackle (neck) where the rooster is more colored thru out. And he has longer pointed hackle and saddle (just before tail) and his tail feathers are longer also. Also notice his comb and waddles are much bigger. He also has the flock leader/ ruler of my domain stature about him. Always watching over his ladies.
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Hopefully some of this is helpful for you. Others will add to this I'm sure.
Also to help for posting reference.
Males under 1 year old are called cockerels, over 1 year old are roosters.
Females under 1 year old are called pullets, over 1 year old are hens.