I usually have chickens that lay different colored eggs, which makes it easier to tell. Different breeds may lay different colors. I've had different shades of brown, tinted (pale), cream, green and blue, some with speckles. Even the ones that lay the same color of egg have differences in the shade or density of the pigment on the egg. I have one right now that lays an egg with less bloom on it, so it's a duller color and not as smooth feeling. Some chickens will lay an egg that's just slightly more rounded or elongated. Others are larger or smaller.
If they stop laying for a few weeks because they're molting or broody, you notice which egg color is missing from the egg basket. Sometimes you happen to be out there right after they come off the nest and you pick up a warm egg. If you hear the egg song from one of them, you can go check the nest for a freshly laid egg. Those are the ways I find out who laid a particular egg.
When pullets have matured enough that they're getting ready to lay for the first time, their comb and wattles will start getting red. When they stop laying because they're broody or molting, the color will go back to pink. When they start laying again, it goes back to red. So, that can help you tell which chicken isn't laying. You can also look at the vent or feel the pelvic bone area, but checking comb color is an easier way.
