Usually there are clues you see or find if they are going to lay an egg. Firstly, look at the face. Is it pale pink or red as is if they have a bad rash. (Lol, thats what i thought they had when they had the red face.) If they are going to lay an egg at that moment, usually i would hear a cry/crow that sounds different.
If you want to make it lay, i suggest has a comfortable environment and has good food with lots of protein, calcium, etc. Make sure it knows where the nest boxes are. It also depends on the breed. Some breeds mature quickly and some dont so just be patient and youll get your egg from her soon. Good luck!