How do you know it's not yours?

if i buy a doppler to hear my baby's heartbeat on my own, how do i know its not my own heart beat that i am hearing?? im not a doctor so i dont think i would know the difference. im 21 wks preggo and i heard it only once at the drs office when i was 12 wks... i dont want to get excited over my own heart beat..lol