I've been using my stethoscope to listen to movements since like 10 weeks. Sometimes it's hard to tell what's what, and sometimes it's really obvious when the baby kicks. Must have to do with the position of the baby, fluids in my body, activity of my digestive tract, etc.
My Dr said that you can start to hear the heartbeat with a stethoscope around 22 weeks. I haven't been able to hear it yet, but you should definitely be able to! I don't know if it would be clear enough to determine the heartrate, but try anyway.
It's great being able to hear and tell where the baby is.. Mine's all the way on the left all the time!