Hi Dana.
If you time the heartbeat you'll be able to know for sure if it's you or the baby. Baby should be somewhere around 160 (a bit lower or higher is okay too). Your heartbeat should definitely not be over 90 when in a resting state, so that's enough of a difference to figure out which you're hearing.
What I do is find little one's heartbeat, then get out the stopwatch on my phone and start counting until the stopwatch hits 30 seconds. Multiply that number by 2 and if it's close to 160 then you have definitely found baby.
I also found it helpful to listen to You Tube videos of other people using the doppler so I knew what type of swoosh swoosh sound to listen for.