The pelvic ultrasound gives somewhat of an overview of what is going on inside. They can look for cysts or fluid on the ovaries, which is what caused my very first bout of spotting a long time ago. They can also look for fibroids, which is what I had mine for last night. It was such a funny experience. I drank all the water they told me to drink for the regular pelvic, but then they needed a closer look transvaginally. They had me empty my bladder, but it kept refilling and they couldn't see my right ovary.
I don't know anything about PCOS, but I think endo can only be seen through a laparoscopy. Not sure, though. I am also guessing they can see other things on the ultrasound.
Total abdominal hysterectomy at age 26 due to abnormal bleeding. Kept ovaries.