How can I tell when a hen is laying or not?

I would like to know if anyone else has this problem. I work a full time job while my girls are at home free ranging. We rescued these hens from a production house so our eggs haven't been as much as I had hoped with having 32 hens. The most eggs I have gotten in one day was 20 (once). We are thinking about getting rid of some of them that are no longer laying and was wondering if there is a way we can do this without sitting in the coop and watching.