Butt check is the way to go (previous poster shared a link)! I’ve used this pretty accurately for 2 of my 4 birds, and am doing it every couple of days for the 2 not yet laying. It’s a good way to gauge if anything is happening down there, along with comb/wattle development and redness. I have a Wyandotte that is 34 weeks and her comb has been red for a while but her pelvis tells me that eggs are still a ways off

but I also have a 25 week EE that is getting close, based on the widening of her pelvis. The other two of mine, I could tell within a few days before they started laying.