(sorry for the mini hijack)
If you hold them backwards and feel where their pubic(not hip-sorry my goof) bones are on either side of their vent, if they are laying or near laying age you should be able to get at least two fingers in between the bones. Non-laying pullets will have narrow pubic bones still-I can hardly get a finger between the bones on my Brahma-she will be a good while yet!
This is way less gross than it sounds if you come from their belly and put your fingers on their soft underfeathers, you don't actually have to touch the vent.

It's a quick way to maybe figure out who has started laying if you get a mystery egg, or if you are just curious. Hope I explained this right, I am new at it myself but it has been helpful.
I also find it funny because when chickens cluck their vent moves. It always makes me laugh to feel their vents moving along to their noises...but I'm easily amused.