I have a variety of chickens - RIR, SLW, EE, BR, Ameraucanas.. Some have laid as early as 20 weeks. It depends on the breed. I find the best indicator is when their combs turn bright red from the pink they had been before.
I put fake ceramic eggs in the nest boxes to encourage them to lay there. Chickens like to keep up with the Joneses, so if they see the nest box being used, then it is ok to lay there. I have had trouble in the past with them not laying in the nest box (mine free range in my yard).
So you may need to keep them cooped until they get used to laying in the nest boxes. I got fake ceramic eggs from Eggnots and they seem to be pretty durable. Cause sometimes the girls kick them out of the nest.
The egg song is that loud and loooong bawking they do after laying an egg. It can go on for two minutes! Here is the link someone posted earlier
Squatting is when they hunch down when you are next to them. The wings go out and up a little. I haven’t found squatting to be a great indicator of laying. My SLW is a great layer and has never once squat for me or the roosters. It depends a lot on the bird. Best indicator I have seen is that red comb.