I have been finding marble size eggs in ckicken run. My father tells me that it is an indication that the hen that layed the egg is ready to start sitting on the eggs to hatch them.
A hen that is broody may also be a sign the hen is getting ready to set. Broody is when the hen is reluctant to leave the nest when you are collecting eggs. She may also peck at you if you try to reach under her and cluck at you strangely as if she did not want to be disturbed. This indicator varies from breed to breed, in some breeds are more broody than others.