The best sign I look for is for her to "wax up". When she looks like she has wax on the end of her nipples, even what looks like a drop of wax from her nipples you are within 24 hrs of delivery. As far as care, just stay out of the way until the foal is out the treat the naval area with an Iodine like solution and then make sure you imprint the foal. Imprinting a foal will save you lots of fights down the road in all aspects of training.
Here is a link to a thread on imprinting that was debated a few years back, it starts out on page 1 mixing up imprinting with ground work on a young foal..... so what I'm saying is (IMO) the first few comments are wrong but (IMO) they get corrected on page 2... so make your you at least read the second page if not all of it.viewtopic.php?f=4&t=33696&st=0&sk=t&sd=a&hilit=imprint&start=15