Usually a mare’s udder will get larger, and there will be some waxing on the teats and possibly a little milk dripping (the droplets are colostrum), but our mare (LA – Lady Afleet) has suffered from mastitis at some earlier point in her life, and it has made it rather difficult to predict. They usually appear to lose muscle and take on a hallow look in the rear up by the tail. There are many websites to help with this. There are even devices to help figure it out. I have no experience with these devises. We watch closely with a camera in the stall and checking every hour when the birth is getting close.