ok.. first.. you don't put your fingers IN the vent.. what you would need to measure is the distance between the pubic bones which can be done from the outside of the bird (and no it doesn't stress them any more than picking them up).
As a hen starts to lay eggs the pubic bones separate to allow for easier passage of the eggs. A bird who is just starting to lay or who is just coming out of moult will have a smaller distance.. the bones do take some time to spread apart.. A hen who has been laying for weeks or months will have bones that are spread further apart than a young pullet who is just starting to lay. I never bother to check a hen during a moult.. there isn't any reason to.. but if she is fully feathered and you suspect her of not laying.. the 3 finger test is a really good way to determine if she is laying or has quit.
You can also check her vent. If it's tightly closed and dry.. she isn't laying.. if it's more open and moist.. she is laying.
Just because the vent is closed doesn't mean a bird is done laying for good.. things to take into consideration are her age.. if she is moulting or has just come out of a moult, her breed, the time of year and if anything has happened to stress them out (like a change to a new home or a predator attack). A chicken's vent can close and they can stop laying if they have been stressed or if it's winter and she is a breed who normally stops laying when the days get shorter.. or if it's been especially hot out.
I never assume a hen is done for good unless she is 3+ years old.. not in moult.. the weather has been good and the vent is dry, closed and i can only get 1 finger between the pubic bones.. then and only then will I cull a hen that isn't producing eggs.
