I've had my Doppler assessments of placental location verified by U.S. One thing I look for in addition to the placental souffle is a cord insertion. When I got one of my Medisonics Dopplers, from Cascade, it came with a bunch of printed info on use of the Doppler. It had a study included on locating the placenta with the dop., and instructions. I found it to be of mild interest, so I didn't exactly memorize it. But I do remember that it gave specific instructions for locating a posterior placenta with the dop.
Posteriors are harder to pin down.
I also find these are the heart tones most difficult to hear with a fetoscope. We determine the approximate location of implantation after the birth. Hole was right ON placental lobe. Can figure out OP and OA, etc., by finding the actual heart. You can hear a difference between cord, placenta and fetal heart.