If they free range they can hide eggs. The only sure way to see if this is the case is keeping them locked in the coop and run for a few days. The difference in browns:
Another way to see who is laying is to check the combs. Laying hens will have combs and wattles that are much redder and more pronounced. A surefire way to see who is laying what is to take a clean finger with food dye and put it on their vent. The hen with the blue dye will have eggs with blue...