A mare's udder at foaling should be enlarged, with colostrum dripping in small quantities. After washing your hands and cleaning her udder, with a gentle soap and water, you can squeeze a teat to make sure colostrum streams from the teat. If you have a mare which foals and she has little to no colostrum, call your veterinarian immediately. Your veterinarian can evaluate the mare for the reason she has no colostrum and might start treatment to help her produce milk. More importantly, if the mare is confirmed not to have enough colostrum, then your veterinarian can help you find a source of colostrum.
Colostrum can be hard to come by.
The best sources are large breeding farms, large private equine practices, and university veterinary hospitals. If colostrum cannot be found, then a substitute must be administered. Equine oral immunoglobulins are available commercially, as is commercial intravenous plasma. Intravenous plasma is more expensive, but I personally prefer it to boost the immunoglobulins in a newborn foal. Your veterinarian can guide you to the best treatment for your foal and for your particular situation.