Can you know if you're having twins with just a Doppler?

If they happened to catch 2 heartbeats aside from your own then you could but if they don't know to check for more then one then they would probably just stop after listening to the 1st one. An u/s would be a much better indicator. If twins run in the family or there is a could chance of twins you should really mention it to your Dr so they can check and be sure..that way if there are they can both be checked at every appt.