Well, a vet can tell you by ultrasound or blood depending how far along she is. Many use the HUG method where you wrap your arms around that belly when they eat their feed or take a cold drink and wait to feel that kick or flutter. Have you a picture of her?
Then I realized I really didn't answer your question...No I do not believe one can hear the heartbeat of an unborn foal. If I am wrong someone let us know.
I too don't believe you can hear the foal's heartbeat. Even if you could, you'd have to have enough knowledge to differentiate between the mare's or the foal's heartbeat. Otherwise, you wouldn't know one from the other.
I think a vet could in late stages of pregnancy, but I've been told that external ultrasound (such as used for sheep) isn't used on minis because the amniotic sac is so much thicker than that of a sheep. I'm thinking maybe that could apply for fetal heartbeat too? Internal ultrasound is different, of course.