Milk will usually start to fill the mare’s udders - often called “bagging up” - a few weeks ahead of foaling. Some mares don’t bag up until very close to foaling. The fluid starts out rather salty and gritty in texture but will change as she gets closer.
When the milk turns to white colostrum (“first milk” with high protein and antibody content) just prior to foaling, the thicker milk often collects at the end of the nipple with a waxy feel to it ... commonly called waxing. Many mares will show wax, other’s will not. Some show wax in quantity, others in very small amounts. The color of the wax (or milk) is the key issue.
The typical process is for the “milk-to-be” to initially change from salty and gritty to clear and bland. During this period any “wax” looks rather amber in color. The mare’s milk then changes to looking more “cloudy” or “watery milk” ... more like skim milk. The change from clear to white is sometimes referred to as transitioning, and may take hours or many days. When thicker and richer white wax or white milk appears, the transition is completed to colostrum.
Different sources provide milk for each nipple. White wax (or white milk) on both nipples would indicate colostrum is ready for the foal. That status may or may not be obvious. Foaling, or parturition, may occur within hours, during the next foaling window, or wait for a subsequent foaling window. (See above regarding the foaling window.)
The traditional method to assess the mare's potential for foaling is to note taste, texture and color of the milk - combined with other evaluation of the mare's behavior and physical status. There are contemporary methods to test the mare's milk to help predict foaling by noting when the milk in both nipples is changed to colostrum based on the Ph balance change.