Is ovarian cancer only detectable by pelvic exam after it has advanced?

Imaging/tissue too. Physical exam and imaging such as ultrasound or ct are helpful in confirming the presence of a mass. Generally the diagnosis requires a surgical procedure to obtain tissue samples to confirm cancer.
Ovarian Cancer. A pelvic exam may sometimes detect an ovarian cancer that is small and confined to the ovary, especially in thinner individuals, but with more abdominal fat and muscle, it is hard to do so, so ultrasound and other imaging techniques, certain lab tests, and cytological and histological sampling are more accurate. The earlier they are detected the better, so, if worried, consult your gyn.