Generally when a cat is ready to give birth, she will start lactating. Some other signs include, her belly dropping,very vocal,hiding,seclusion. There will also be a discharge very shortly before labour.
There are a few sign a cat will give when It's ready to give birth. The cat will secrete milk from the nipples and six to twenty-four hours before she is ready to give birth she will start to pant,shiver,pacing around or act anxious. These are all signs that she is close.