Typically, baby's heartbeat should be ale to be heard beginning right at 20 weeks. Though admittedly, it does take some skill and patience to maneuver that so early on. Back in the olden days, pre-ultrasound popularity midwives would use when they could auscultate the fetal heart tones as a way of confirming dates.
As the pregnancy progresses, the key to finding clear fetal heart rate sounds is to first identify baby's position and then listen on the upper part of baby's back. Midwives also used to, and some still do, use listening with a fetoscope as a way to confirm baby's position to see whether they are breech or not. Doing so tends to yield very accurate results.
Quality varies. Shorter ones tend to have clearer sound than longer ones.