My friends mom had 2 sets of twins and with her first set she did not know she was carrying twins until she gave birth. They gave her an ultrasound and said it was a girl but they later found out of course that one was hiding behind the other.
The proper ultrasound can reveal this. If there are twin it will become very easy to find out their position and condition. It is possible that twin can have one heart if they are identical twin, it means if they are in one placenta.