A woman complains of having slight dark bloody discharges and mild pains in the lower part of abdomen for several days. Last menses were 7 weeks ago. The pregnancy test is positive. Bimanual investigation: the body of the uterus indicates for about 5-6 weeks of pregnancy, it is soft, painless. In the left appendage there is a retort-like formation, 7x5 cm large, mobile, painless. What examination is necessary for detection of fetus localization?