If they are Silver Cuckoo Marans, then the males will be much lighter...and have far less feathers, and usually kinda bald almost on their back...and much lighter, almost silvery gray.
The cockerels will also not develop a tail for a while.....but they will get much larger shanks & feet after a while it is so easy to see by their baldness/gray fluff/big shanks ..The pullets will have 2 distinct rows of flight feathers on their wings, almost so they are sticking way out on the wingtips as if over grown for the body size...they are much darker, have a smaller dot on the head, and develop feathers all over the body (and back) fast...that one above looks like a pullet.