When looking at the signs of imminent foaling, it is important to remember a couple of things.
Firstly, mares do not read websites nor books, so just because a website or book says a mare will/will not do something, does not mean that will be the case - when it comes to breeding, we can only give generalisations i.e. a particular thing often happens to most mares, but cannot be guaranteed to happen with every mare. Some mares will show all the classic textbook signs of foaling, some will show some of the signs, other mares will show none at all. An old breeder once said to us that the only true sign of knowing a foal is definitely coming is when you see a pair of feet sticking out the back of a mare - how very true that is!
Secondly, due dates are an estimate, not a guarantee. Mares may foal up to around 3-4 weeks either side of their apparant due date, and still be considered normal. Some mares will foal early every time they foal, others late every time, others like to keep you on your toes & vary it. Length of gestation is usually taken as being 340 days.
The following are signs of mares preparing to foal, which mares may/may not show:
Bagging up - a mares udder will start to fill up 4-6 weeks on average before the birth. However some mares will not bag up until they are actually in labour, or even after! Generally the position of the teats can give you some idea of how the mares preparations are continuing - initially they will point inwards, and as she gets more ready, they will begin to point downwards, and may even point out, in addition, the teats themselves will begin to fill, and have less definition from the bag.
Waxing up - this is a waxy white deposit which appears on the end of the teats - usually seen in the very last few hours/days before labour begins.
Running milk - mares may actually begin to drip fluid down their legs, sometimes just when moving, sometimes if just standing. If this is excessive, capturing & freezing some of this fluid should be considered, as this is likely to be colostrum which is needed by the foal shortly after birth, and this can be bottle fed back to the foal after birth.
Changes in colour & consistency of milk - milk expressed will change in colour & consistency. It will go from being clear to being white, and in the last hours will become sticky & creamy. It is possible to buy milk predictor kits which will test the calcium levels of the milk, which can help you to predict the day of birth.
Change in shape of mare - as the foal changes position, in preparation for birth, the shape of the mares belly will also change. The belly may become lower, less wide, and drop away from the hips a little, and sometimes may even have a slightly pointed look underneath.
Loose muscles - the muscles around the mares quarters near the tail will loosen. The area around the tail may begin to look hollow. This can happen up to two to three weeks before birth.
Behavioural changes - this can vary with each mare. It really comes down to knowing your mare - what you are looking for are not so much specific behaviours, but rather a change from your mares normal behaviour. Some mares may chose to go off away from others on their own, others may be quieter, some may be grumpy - what you are looking for is a change in the behaviour of your mare from her usual routine/mood.
Fleshy & lengthened vulva - the mares vulva relaxes & lengthens in preparation for the coming birth. The creases around the vulva will often fill in, and the slit lengthens. However this observation needs to be done carefully, as often on lifting a mares tail, she will tense, giving you a false picture of what is going on.
Final Signs - in the last hours before labour, mares may do a number of small sloppy droppings, be restless, get up & down regularly, sweat up, kick at their bellies, stand as if planning to urinate without actually doing so, keep their tail raised, become grumpy & not want to be near others, pause in the act of eating for a few minutes and stand still before continuing eating, paw in their bed. Once you begin to see these symptoms, it is likely that your mare is in first stage labour, and her foal will soon arrive.