I can tell you about what I use fetal heart tones for. I may me more medically minded, but perhaps some other midwives will be along to describe their practices, too.
At a regular appt, I generally just listen with a doppler for a minute or so. I consider reassuring hearttones in that setting to be simply a rate between 120 and the 160s (or even a little higher in the first or second trimester.) I have rarely heard something concerning, and then sought further testing. Some examples of something concerning heard just on doppler in a regular appt setting would be tachycardia (rate over 170) that doesn't come down with listening for a while (frequently, I hear a baby who is moving around, or bothered by me poking at him/her whose rate will go up for a minute or so, and as I listen it comes back down -I don't worry a bit about that.) Bradycardia (rate under 120, especially under 110) would be another example, especially if it doesn't come right back up. Occasionally, I hear a brief drop in heart rate, usually associated with the baby changing position, and I don't worry about that, either. A final example of non-reassuring heart tones heard in an appt setting would be an arrythmia. Most fetal arrythmias are in fact harmless and resolve after birth, but some can be a sign of a serious problem, so usually I would seek further testing for that, too.
For all of these situations, the first further testing I would be looking for would be a non-stress test. Getting a continuous recording of the heart rate is more accurate than listening for a minute or so, so if the non-stress test is reassuring (normal baseline rate, no heart rate decelerations, and 2 accelerations within 10 minutes) I would be reassured. For tachycardia and bradycardia, if the non-stress test was good, I would look no further and probably take no action - unless there were other reasons to consider an induction, in which case the slight concern over heart tones might tip the scales in that direction. For an arrythmia, I would likely proceed to ultrasound to make sure the baby's heart appears normal and there are no signs of other illness - and possibly to fetal echocardiogram if there is reason to believe there may be anything wrong with the heart.
In labor, I often monitor intermittently (and that is my standard order.) We use either the doppler to monitor intermittent, or if the mom is near the monitor, we use the fetal monitor to listen briefly. There, I am looking for heart tones before, during, and after a contraction, and looking for a normal rate (120-160s) with no significant deceleration. Accelerations, if they are heard, are reassuring - but if not, in labor I'm not terribly worried about accels, unless there are other signs the baby isn't doing well. A significant deceleration is usually at least 10 beats per minute lasting 10 seconds or more. On a fetal monitor strip you can see more subtle decels, but they are not as concerning. If I heard a decelerations, I would first listen longer - through the next 2 or so contractions, and if all is normal, I would usually return to intermittent monitoring. If I'm still concerned, I'd switch to the continuous monitor, at least for a while. On continuous monitoring, I'm concerned if I see late decelerations (decels that start after the peak of a contraction) because they are most predictive of baby not getting enough oxygen - or if I see repetitive variable decels (decels that occur with variable relationship to contractions) that are prolonged and take a long time to recover. I don't worry at all about early decelerations, which mirror the contractions, starting before the peak of a contraction and resolving as the contraction ends. Those are caused by fetal descent and head compression and are a good sign that the baby is moving down. For concerning heart tones, my first line of action would be to make sure the mom is not on her back, trying several positions, make sure mom is hydrated, make sure mom is not hypotensive (especially with an epidural mom) and make sure we aren't causing too many contractions for the baby to recover from (as in when giving pitocin) For persistent tachycardia, I also look for hypotension (seen more often in my practice with epidurals) and fever. If fever can be reduced and BP corrected, tachycardia often resolves. I sometimes see tachycardia in the end of second stage, too - I'm guessing from the baby's stress hormones, and usually we just wait that out.
If none of that works, then you have to decide how much is too much, how low is too low, and basically guess when you think the baby might be getting to a point where it is distressed and doesn't have enough reserve to recover easily following birth. A few other markers can help you know the baby is still okay. One is fetal scalp pH, which isn't available many places. Another is to stimulate the baby by rubbing it's head, and if there is a resulting acceleration, that is as reassuring as normal scalp pH. Other than that, you have to take into account how far along the mom is, how severe the decelerations are, and how soon the baby is likely to be born - all of which, are of course, estimates. I'd be less likely to act on decels seen with a 5th time mom at 9 cms, than a first time mom at 4 cms.
Is any of that helpful?