One way to tell if a pullet is laying or is about to, is the redness of the comb. Another way to tell is to locate the bony protrusions on either side of the vent. If you can fit two fingers in between the "nobs", that pullet should be ready to lay.
Another way to determine which egg was laid by which hen is to deposit a bit of food dye just inside the vent, a different color for each layer, and when the egg is laid, being wet, it picks up some of the dye on the shell.
Or you can hang around and peek into the nest boxes, seeing who's in there.