I found this website to have a lot of useful photos, especially once my mare got close. Foaling
I also bought "The Complete Book of Foaling" and a friend gave me the book "Blessed are the Broodmares." Both gave me lots and lots of good info. I would suggest getting at least one good foaling book.
Both are short of photos though, that's why the website link I gave above is nice. You can actually see what the milk changing color looks like.
From what I can tell, from my first and only foaling experience, the consistency and color of the milk is your biggest guide. My girl had clear, watery "milk," then clear sticky "milk" and then finally about two days before she foaled, she had actual white milk that leaked easily from her udder. But I know the timing can vary alot from mare to mare.
But anyway, good luck with your girl! My foal was a rescue of sorts too. I actually bought my mare (a Foxtrotter) really cheap as my next trail horse and it turns out she was pregnant. I was really happy about it though, because I always had a fantasy of having a foal.
My mare's foaling actually went great, I missed it by about 1/2 hour and mare and foal were doing well when I discovered them (baby was standing but still sopping wet!).
Then we had sort of a rocky start because he got a navel infection due to a patent urachus (urine leaking from the navel) but all seems under control now and he is a strapping, almost 3 week old colt whom I adore.
Anyway, another book I really like is "Imprint Training of the Newborn Foal" by Dr. Miller. I didn't do the imprinting exactly per the book, and I missed handling my foal before he stood, but I still touched him all over several times the first day, (and had to hold him down to doctor his navel for about a week, sigh!) and anyway, he is a very friendly foal and very easy to handle thus far. Plus, the book has lots of good info on halter training and such. So even if you don't "imprint" per say, it gives you lots of good info on training your foal.
I have my guy leading fairly well, picking up his feet, and moving off of pressure from my fingertips and he is not quite three weeks old. I am working on tying and ponying him too.
Sorry for the novel, but I hope that helps a little.
PS. Another thing I would suggest is really getting to know your mare before she foals, and handling and grooming her a lot and really getting to know her and what she is like temperment-wise. Because if you want to be able to handle and work with baby, you need to be on good terms with momma!
My mare is sweet as can be, but was/ is very protective of baby. Luckily that mostly shows up when other horses are around, but I still keep a good eye on her because she is rather irritable since she had her foal. She threatens other horses a lot if they get too close, and even threatens baby if he is too pushy nursing. So try to build a good relationship with your mare before baby is born.