In this time of U/S exams being done with fair frequency, the chances of a hidden twin not being spotted is much smaller than it used to be. Using a doppler to detect the heartbeat also is a way to detect a 2nd little somebody inside. Early on, your beta levels would be likely to be higher and that would indicate the possibility of twins. If your beta levels were normal for a singleton and no extra shape or heartbeat has been detected, the chances of that hidden twin are probably small. Your next U/S should tell for sure. In fact, tell the U/S tech that no twin has been found yet, but since twins run in both sides of the family, you'd like the U/S to be extra thorough just to answer that question.
Hope you get the answer you want!