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.