This doesn't sound like a good solution for you, but we listened for the 'song of triumph' from our girls and ran out to the coop to see if it was real or if she was just faking us out.
We eventually learned what their eggs looked like, who layed when and where and how often.
When your girls are free ranging, do any of them make nests for themselves? My Black Star did, so we always knew which eggs were hers. Eventually I figured out that she kept a strict schedule: 1 egg a day on week days, Saturdays off, and occasionally an egg on a Sunday if she felt like putting in a little overtime.
Edited to add: I have read that sheep farmers put colored chalk on their rams chests to mark the ewes they have mated. Maybe you could get some and try it along the nest box like you mentioned?