About the nest poop, make sure their roost is higher than the nest. For safety, they always want to roost & spend the night at the higher spot and they poop a lot during the night. So, most people try to put the roost a little higher than the nest boxes to discourage sleeping there.
About what to look for to know if they are about to lay:
1. 15 weeks is a little too young yet. My earliest bird laid at 19+ weeks, the rest laid between 24-30 weeks, with one very late layer at 39 weeks.
2. I'd look for VERY developed & red combs and wattles, plus very red faces. (check pics on BYC for examples)
3. Do they squat, when you reach for them? If they do, they are maybe a week or two from laying (put out oyster shell for them)
4. If you check the pelvic bones spacing on their side of the vent, for large breeds you usually can feel a space big enough to fit 3 fingers. If it's narrower than that, they won't be laying for a while.
5. Interested in the nestboxes -- spending time fussing with the straw in the nest, and maybe sitting there, like they're gonna lay.
6. Walking around, tossing straw over their shoulders (means they are REALLY close to laying their egg - so expect one THAT day!)
Put a golf ball into each nest, that will stimulate their desire to lay there- when they're ready to lay. Other than that, feed & water them well and wait, wait, wait...