According to the most recent guidelines for the management of endometriosis by the European Society of Human Reproduction and Embryology (ESHRE), the only way to have a proper diagnosis of endometriosis is histological analysis on the surgical specimen. You can *suspect* the presence of endometriosis by anamnesis and physical examination (vaginal or rectal, in case of virgo), and by ultrasound in case of endometriomas or deep infiltrating endometriosis, but the only way to exclude the presence of superficial endometriotic foci is diagnostic laparoscopy.
All the possible lab tests, including the dosage of CA125, have no reliable sensitivity and specificity to predict the presence of endometriosis (in particular, you can have high CA125 in I-II rAFS stages and, paradoxically, low CA125 in III-IV rFAS stages).
Yes I guess that teenage develop endometriosis when present algodismenoree severe even at menarche. According to the recommendations Congress on Endometriosis I, treatment with COCs is the binefic.