What breeds do you have?
I know which of my chickens are laying. They would squat and of course I could fit 3 fingers between their pelvic bones...disgusting but accurate.
You can tell by using the chickens pelvic bones. It should be under the chickens vent. If you can fit atleast 3 fingers between the bones, that chickens is laying, if not its not laying. Also do all your chickens lay the same color?