As the birth gets closer, you should notice her vulva swell and elongate.She will get milk in (generally two days before birth, but since this is biology, that doesnt mean it is 100% accurate always) and right before birth you might see a waxy coating on the ends of the teats. Usually the day of, or the day before she will not eat very much, or even nothing at all. She will seem restless and definately try and make herself a nest. As it gets closer she will start breathing more often. She will expell a mucus plug (anywhere from a few days to even only a few hours before birth.) Of course an obvious sign will be you seeing her sides almost twitching from contractions.
good luck.. and we want pictures of the babies!