I heard both my girls around 6-7 weeks with that vaginal wand that they use in the office. I was able to hear on the doppler at around 12 weeks like michellH said.
We heard our babies heartbeat at 6 weeks. My doc checks baby with ultrasound right in her office and was able to show us and let us hear the heartbeat at our first visit at 6 weeks but other then that with Doppler its about 12 weeks it could be picked up a little sooner but its tricky