I've noticed with mine about three weeks before the hens start to lay, the rooster leads them around and shows them places to lay their eggs. I did not have the nest boxes built yet, so the looked all around the deck, under things, behind planters etc. They had a big discussion over each spot. While the hens watch (two or three hens at a time) the rooster gets in the spot he's found, sits in it and swooshes his hinny around like he's laying an egg and demonstrating how the location will work!! The hens say something whiny like they don't like the spot and move on to another spot and discuss it, the hens looking skeptical. I would also see nearer the time they started to lay, single hens going around like they were searching for something. I got busy and built nest boxes. They all came and discussed the location, got in it and tried it out, and decided it would do nicely.
It is good not to make the nest boxes available until they are in the habit of going on the roosts to roost. Nothing worse than reaching into the nest box to get an egg and sticking ones fingers into a big gob of nightly poop.
My Comets started laying at 18 to 20 weeks - so at 15 weeks have the nest boxes ready.