how to tell if hen or cockerel

Dark red combd,and wattles,bigger then the hens.

 

Knowing the breed helps. Photos are useful too.

At 5 weeks and up cockerels do start to show red color in combs. Many and dare I say most chickens are feather sexable. There's a bit to feather sexing but the quick and easy is to note which birds have wing feathers first. It's usually a huge difference between the sexes with the pullets growing wing feathers first. Fuzzy head and body with feathered wings. Your at the age to be seeing this.

From hatch noting the leg size is fairly accurate too. Large legs-cockerel.

 

X2 on Egghead Jr. A bird at this age with a large red comb would likely be a cockerel. But, it gets easier to determine gender as birds get older- by 14 weeks or so, males will be developing pointy saddle feathers and hackle feathers.