Quote: It's tough to know who is laying what, unless you can sit in the coop all day long and watch.
I've read that gel type food coloring on vents works pretty good.
Trap nests is the only sure fire way that I know, but that has it's own challenges...including being around to release hens.
A good surveillance camera might work too, but then you've got several hours of video to review and even then might be hard to see who left what in what nest.
How many birds do you have?
Do you have separate enclosures that you could segregate the hens in? Tho that presents other challenges.