Why do you ask?? Has something happened? Are they inside or you do help a homeless outside??
It is not usual they stop giving milk, unless directly starving or sick. But it happens they dont get enough.
The best, perhaps even only, way to know is to weigh them every day. Almost all breeders do so, and most rescuers too - even with semiferale moms.
Use a kitchen/mail scale, weighting in grammes. Easiest with electronical scale. They dont need to be costly, otherwise you can surely borrow one from some neigbour... They should go 10+ grammes almost every day...