Before I set my breed pens, I begin checking for egg laying.
Like BeeKissed, I have checked with gloves doing the rectal exam....For large fowl, use my index finger only to the first knuckle. For my Bantam hens, I use my pinky finger.
The hens are not uncomfortable because the pressure is very gentle. The egg is ready for laying and so far, I have never broken an egg.
As a precaution, I dip my gloved fingers in a 10% Oxine solution between hens. Oxine is anti-fungal, anti-bacterial and anti-viral.
Another way to identify a laying hen instead of trap nesting is to use food coloring in the vent...a drop is plenty. When I have a trio or quad and want to know who is laying what egg, I apply a different color to each hen in the morning. When she lays her egg, the color is streaked on the egg.
Once I know the shape and shell color shade for each hen, I no longer need the food coloring.
This helps when one or two hens are laying or eggs are not fertile or vital during incubation.
My last tip, a hen that is laying will have a moist, pliable vent while one that is not laying will have a shrunken dryer vent.
When culling for laying, it is best to check the girls when peak laying is occurring.
Not good during molt or while a hen is brooding chicks or when you have moved your hens to new pens, given them a new rooster, or your weather is too hot or too cold.
Many things can affect laying...telling them to get busy or become soup, just won't do it.