You can put one hen in a dog crate or similar pen, and leave her there until she lays an egg--then make a note of who she was and what it was. Repeat as needed. (Put one in really early in the morning, and you may have your results that day.)
Or, pick a time when you only get a few eggs, then go look at each hen's vent to see who's laying. That's especially useful if you've got pullets just starting to lay. When you get the first egg of a particular color, go check all the pullets that it could have come from. (Works best when they start at different times, less well if they all start at once. Different breeds tend to mature at different rates, and even individuals of the same breed can mature at different rates.)