I've only had one mare foal, so I'm not much help either except that two days before my mare foaled, her milk turned white, like cow's milk. And the night before she foal she only nibbled at her hay. Obviously the not-eating thing isn't a good indicator for your mare, but I think the milk color IS a good indicator.
Before my mare's milk turned white, it was just a clear-to-cloudy color. But when it was really, really white she foaled withing two days.
Of course be careful because your mare may not appreciate being "milked" but my girl was fine with it. I got the milk color info. From a book, but there are also internet sites that tell you more about that. I didn't save the links since my mare already foaled or I would post them for you.
The book I have, which is really awesome, is called "The Complete Book of Foaling" by Karen Hayes, DVM.