A sonogram can very easily see an ovarian cyst. A transvaginal sonogram can even pick up your egg follicles. A doppler assisted sonogram can track the blood flow in the ovaries and if there is any blood flow to the cyst.
If you had a simple cyst that resolved itself, sometimes the amount of "free fluid" in the area is increased because that was once what was filling the cyst. If you had your cyst open a week ago the free fluid will have already been absorbed by your body.
If you have a tumor all types of sonograms can pick it up. Unlike a breast sonogram, a pelvic does not miss things. They clearly see the ovaries, uterus and even the thickenss of the uterine lining.