stage is determined during surgery because that is when a more accurate determination can be made.
There is no way to confirm cancer, or to stage it, until surgery is done and a complete pathology done. Make sure that your mom's surgery is done by a gynecological oncologist, as they are the experts in dealing with any sort of ovarian cyst, whether or not it's cancer. If it is cancer, they are best at doing a proper staging, which is essential for determining the best & most effective treatment.