Sarah, I always say the only way to be 100% sure is when the piglets are running around - pigs have a habbit of being, well, pigs and aim to surprise us
The udder will start to form around a month before farrowing, from 10 days to a week before farrowing her vulva will start to swell up and get redder. This is the time, if you hadn't done so already, to seperate her from the rest into her farrowing area and worm her (I try to get mine in and wormed 2 weeks before farrowing)
Milk will (sometimes) start to come in around 2-3 days before (if you give her teats a gentle squeeze you'll see droplets) and by the time the milk is really in (coming out without stimulation) you're about 12-24 hours away.
HTH
Karen