How do you know when they are about to lay? The only thing that I know is that they lay late in the evening.
Honestly, If I were you I would not focus on trying to see it happen. I can guarantee you that if a hen is going to lay it will be around 30 minutes before dark and every other day. I avoid the pens during this time. If I disturb most of them during this time it is almost certainly going to result in a egg dropped from the perch instead of layed on the ground.
Every time I have seen an egg layed, it happens almost immediately without warning. I have a group of midnight birds that free ranges during the off season and is penned during breeding season. The two hens will lay right in front of me. One second they are walking around the next second there is an egg after a short squat. They just kind of look like they might be hunting for a spot to sit or food to eat. Neither of them make much noise at all.