How to id who is laying eggs

Well, we had band practice last night, so no new work on the plucker.  Instead, I will share a neat little trick that I got from our Chicken's 101 group.  If you think one of your girls is holding out, how do you tell which one?  Take food coloring and put a couple drops in the cloaca of your hens.  The hens will lay eggs streaked with the color you put on them.  If you have a bunch of chickens, this may take some time, but what an ingenious idea.