Yeah, you read about that all the time on here, as if it is a lead pipe cinch and every chicken in the world does that. I hate to tell you but it isn't so.
I had a 13-week-old pullet willingly squat for a 13-week-old cockerel. She did not lay an egg for at least 6 more weeks. It was not a sign for her. My pullets or hens practically never squat for me, but I generally have roosters in the flock. Some of my pullets and hens willingly squat for the roosters. Some of these are laying eggs, some are not but they might squat anyway. Some are not laying eggs, these might or might not squat. Squatting and other parts of the mating act are often more about dominance and social relationships than fertilizing the eggs.
Having said that, squatting can be a sign that a pullet is about ready to lay. I consider it a good sign, better than not squatting. The combs and wattles turning bright red is a good sign one is getting ready to lay nut not a guarantee. A pullet scratching in a nest is a good sigh that she may be getting ready to lay. There are several sigs that a pullet may be about ready to lay, but none are an absolute guarantee.
One really good way to me to check which one is laying or at least getting real close is to look at the vent. If the vent is tight and dry, she is not laying and won't be for at least a few days. If the vent is loose, pink and looks moist she is either laying or real close. Once you see the difference it's real easy to tell the difference. That won't tell you how often she is laying, but it is the best indicator I know of to tell which one unless you actually see the egg pop out.