You'll know it when you see it. Mine will stomp their feet quickly, do a few head twitches, bow their backs, lift their tails, and then they'll space their wings out. Do a little pat on the a back and then they'll stand up and fluff themselves.
One of my will scratch a my feet and peck my legs until I pet her. As soon as I start to bend over, she squats and spreads her wings for me. She loves attention...
When they are at the age that they are ready to lay they may hunker down when you go to pet them. They see you as a rooster ready to get cozy. I say that they may squat, because my EEs never did, even though my BO and BR have and do.
OH, my friend's barred rock did this when I was in her pen petting her once. Right before she started laying. I thought she was just being friendly and submissive. didn't realize it had to do with mating behavior. hmmm. So far I haven't seen my chickens do this, although it looks like someones been hanging out in the nesting box- the shavings are all flattened in the middle. Should be within the next several weeks. I think they're 18 or 19 weeks now.
Usually it's a good sign that they are about to lay but I have a BO that lays almost every day and has never once squatted? All of the rest of them do. I've had one that squatted for an entire month before she started laying. Some of them are really funny about it. They'll run in front of you just to stomp those feet and squat.
They put their legs apart and spread their wings but still folded - almost like they're hot - and they hunker down and bow their head down and stay still. Like someone else said, if you give 'em a bit of a pat they get all happy and do a ruffle dance, or as I call it, the cigarette dance.