babies heart beats are around 140-160 bpm so if you try counting how many beats during a minute on your watch you'll soon see if it's yours or baba's - good luck and congrats!!! :-)
I just bought a doppler but thankfully one with an LCD display of the heartrate it detects. I find it so hard to find the heartbeat and keep getting mine!! have you heard it with your midwife yet? that helped me know how fast it should sound.
Mine too has an LCD display but it kept displaying different heart rates. My Midwife happened to phone the day I got it so she came around to let me listen with her doppler. Once I knew what I was listening for it was a lot easier to find.