Check comb and waddle color. Bright red means laying, pale means not.
If they are squatting they are getting ready to lay or are laying.
Are they scratching around in the nest boxes during the day and practicing in them? That’s another sign.
You can also check how wide their pelvic bones are. There is a great article on this site about how to do it.
https://www.backyardchickens.com/articles/who-is-laying-and-who-is-not-butt-check.73309/
Lastly I have never done this but I’ve heard of people putting food dye on the girls vents so that they could ID who is laying based on the dye color left on the egg.
Hope this helps!