There is some science behind it but things like comb/wattle color, whether the vents are soft, whether they squat are clues, not absolute signs they are laying. Bright red combs/wattles are a sexual sign to the rooster that the hen’s eggs might need to be fertilized so hens laying often have bright red combs/wattles. But the color or brightness of the comb/wattles can change throughout the day. And some just don’t get that bright.
A hen that is laying, about to lay, or just stopped laying should have a large, moist, soft vent. It’s not a sure sign she is actually laying but a small, hard, dry vent is a sign she is not laying. That is a good way to eliminate a suspect more than confirm one is actually laying.
A pullet that willingly squats for a rooster (or maybe you) is a sign she is sexually maturing and may be close to laying, but I’ve seen a 13 week old pullet willingly squat for a cockerel two months before she laid her first egg. That was about dominance, not sexual maturity.
There are several signs but the only proof a hen is laying is what she leaves in the nest. I almost forgot, the width of the pelvic bones is another sign, wider bones means she is closer to laying.
I’ve had pullets start to lay at 18 weeks, I’ve had pullets lay their first egg at nine months, my only blue egg layers so there was no doubt which ones laid those eggs. There are a lot of different things that influence that. Heredity is a big one but whether the days are getting longer or shorter, diet, is your weather severely cold or hot, day length, and who knows what else all have an influence. Interestingly the ones that started laying at nine months started the first week of December when my days were about as short as they were going to get and still getting shorter. I did not add additional lights. That goes against all the rules. When they lay their first egg is really up to the individual pullet.
I also leave one golf ball in each nest permanently. It helps show them where to lay. I’ve seen a hen lay on the coop floor next to a golf ball that got knocked out of the nest. I have no doubt the balls encourage them where to lay. I have had hens start to lay in odd places when the golf balls are missing. A couple of times I’ve had a snake enter the coop overnight and eat the golf balls, thinking they were eggs. I don’t know if a snake can regurgitate the golf balls or not, I’ve read both ways, but that’s supposed to kill them. It did in my case because they could not get back out through the holes they came in, the golf balls would not fit. But one member of this forum that I trust said her hens would not lay in her nests until she removed the fake eggs. When you deal with living animals you just don’t know what will actually happen.
I have three nests. Yesterday I got five eggs total, either one or two eggs in each nest. Most days I’ll get most of those eggs in the same nest, usually the upper nest. But there are days most of them might be in the lower left nest. Some hens are very strong in laying in the same nest every day, others move around.
People like to think there are hard and fast rules on how a living animal will behave or produce, but each really is an individual. You can’t be too rigid in thinking about them, you have to remain flexible.