Can a doctor tell from a pelvic ultrasound if an ovarian cyst is cancerous or benign? or do they all look alike and need further tests?

It depends: Certain features make it highly likely that a cystic lesion is benign(e.g. a small "simple cyst"), and others make it more likely to be malignant. There is significant overlap between benign and malignant features, so at times it is impossible to tell. Other times the appearance is so characteristic that benignity/malignancy can be predicted fairly reliably. Age and other findings also factor in

Sometimes: Ovarian cysts are common and the majority of them are benign. A small simple cyst in a menstruating woman is almost undoubtedly benign. More complex cysts may be benign or malignant, and follow-up is necessary to see what happens. (There were guidelines published in Radiology magazine about 4-5 years ago.)