This is something that I have also been thinking on in case I end up doing my partial-unassisted. I'd want a waterbirth so is a doplar better for that? Or can you use a fetoscope as long as your high enough out of the water that your belly's not under?
Here is the section on FHT monitoring from Ina May's Spiritual Midwifery:
"Baby's heartbeat should be checked and recorded every half hour during the first stage [of labor]. Always check if you have even a slight question in your mind about how the baby's doing. Then you can either dismiss the question or fix the situation. The normal heart rate is 120-160 beats per minute. The heart rate becomes slower at the onset, or sometimes the height, of a rush [Ina's term for contractions] and returns to normal by 10-15 seconds after the rush. A heart rate of less than 100 or more than 160 bpm with the uterus at rest suggests that the baby is in trouble. The baby's heart rate is somewhat harder to hear during a rush because the uterus is thicker, so check the heartbeat between rushes. A marked increase in the heart rate is the first sign of hypoxia, and also a sign of possible intrauterine infection of the baby. A slow heart rate or one that does not recover to normal after a rush always indicates fetal hypoxia and can be a sign that the cord may be compressed. Changing the mother's position especially if she has been lying on her back, often helps the baby's heart rate return to a normal pattern."
Btw, I would highly recommend reading that book. I just ordered her Guide to Childbirth as well.